Add `dead_on_return` attribute, which is meant to be taken advantage
by the frontend, and states that the memory pointed to by the argument
is dead upon function return. As with `byval`, it is supposed to be
used for passing aggregates by value. The difference lies in the ABI:
`byval` implies that the pointer is explicitly passed as argument to
the callee (during codegen the copy is emitted as per byval contract),
whereas a `dead_on_return`-marked argument implies that the copy
already exists in the IR, is located at a specific stack offset within
the caller, and this memory will not be read further by the caller upon
callee return – or otherwise poison, if read before being written.
RFC: https://discourse.llvm.org/t/rfc-add-dead-on-return-attribute/86871.
In Ada, a record type can have a non-constant size, and a field can
appear at a non-constant bit offset in a record.
To support this, this patch changes DIType to record the size and offset
using metadata, rather than plain integers. In addition to a constant
offset, both DIVariable and DIExpression are now supported here.
One thing of note in this patch is the choice of how exactly to
represent a non-constant bit offset, with the difficulty being that
DWARF 5 does not support this. DWARF 3 did have a way to support a
non-constant byte offset, combined with a constant bit offset within the
byte, but this was deprecated in DWARF 4 and removed from DWARF 5.
This patch takes a simple approach: a DWARF extension allowing the use
of an expression with DW_AT_data_bit_offset. There is a corresponding
DWARF issue, see https://dwarfstd.org/issues/250501.1.html. The main
reason for this approach is that it keeps API simplicity: just a single
value is needed, rather than having separate data describing the byte
offset and the bit within the byte.
Start removing debug intrinsics support -- starting with the flag that
controls production of their replacement, debug records. This patch
removes the command-line-flag and with it the ability to switch back to
intrinsics. The module / function / block level "IsNewDbgInfoFormat"
flags get hardcoded to true, I'll to incrementally remove things that
depend on those flags.
Reapply PR142507 with fix for test: add in the same x86_64-linux
requirement as other tests as the stack ids are currently computed
differently on big endian systems. This will be investigated separately.
In order to allow selective reporting of context hinting during the LTO
link, and in the future to allow selective more aggressive cloning, add
an option to specify a minimum percent of the max cold size in the
profile summary. Contexts that meet that threshold will get context size
info metadata (and ThinLTO summary information) on the associated
allocations.
Specifying -memprof-report-hinted-sizes during the pre-LTO compile step
will continue to cause all contexts to receive this metadata. But
specifying -memprof-report-hinted-sizes only during the LTO link will
cause only those that meet the new threshold and have the metadata to
get reported.
To support this, because the alloc info summary and associated bitcode
requires the context size information to be in the same order as the
other context information, 0s are inserted for contexts without this
metadata. The bitcode writer uses a more compact format for the context
ids to allow better compression of the 0s.
As part of this change several helper methods are added to query whether
metadata contains context size info on any or all contexts.
In order to allow selective reporting of context hinting during the LTO
link, and in the future to allow selective more aggressive cloning, add
an option to specify a minimum percent of the max cold size in the
profile summary. Contexts that meet that threshold will get context size
info metadata (and ThinLTO summary information) on the associated
allocations.
Specifying -memprof-report-hinted-sizes during the pre-LTO compile step
will continue to cause all contexts to receive this metadata. But
specifying -memprof-report-hinted-sizes only during the LTO link will
cause only those that meet the new threshold and have the metadata to
get reported.
To support this, because the alloc info summary and associated bitcode
requires the context size information to be in the same order as the
other context information, 0s are inserted for contexts without this
metadata. The bitcode writer uses a more compact format for the context
ids to allow better compression of the 0s.
As part of this change several helper methods are added to query whether
metadata contains context size info on any or all contexts.
## Purpose
This patch is one in a series of code-mods that annotate LLVM’s public
interface for export. This patch annotates the `llvm/AsmParser`,
`llvm/BinaryFormat`, `llvm/Bitcode` and `llvm/Bitstream libraries. These
annotations currently have no meaningful impact on the LLVM build;
however, they are a prerequisite to support an LLVM Windows DLL (shared
library) build.
## Background
This effort is tracked in #109483. Additional context is provided in
[this
discourse](https://discourse.llvm.org/t/psa-annotating-llvm-public-interface/85307),
and documentation for `LLVM_ABI` and related annotations is found in the
LLVM repo
[here](https://github.com/llvm/llvm-project/blob/main/llvm/docs/InterfaceExportAnnotations.rst).
The bulk of these changes were generated automatically using the
[Interface Definition Scanner (IDS)](https://github.com/compnerd/ids)
tool, followed formatting with `git clang-format`.
The following manual adjustments were also applied after running IDS on
Linux:
- Add `LLVM_ABI_FRIEND` to friend member functions declared with
`LLVM_ABI`
- Add `LLVM_ABI` symbols that require export but are not declared in
headers
## Validation
Local builds and tests to validate cross-platform compatibility. This
included llvm, clang, and lldb on the following configurations:
- Windows with MSVC
- Windows with Clang
- Linux with GCC
- Linux with Clang
- Darwin with Clang
These are identified by misc-include-cleaner. I've filtered out those
that break builds. Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.
With this change, some callers get to use StringRef::starts_with.
I'm planning to teach getAsmString to return StringRef also, but
I'ld like to keep that separate from this patch.
Since we currently only use the context information in the alloc info
summary in the LTO backend for assertion checking, there is no need to
write this into the combined summary index for distributed ThinLTO for
NDEBUG builds. Put this under a new -combined-index-memprof-context
option which is off by default for NDEBUG.
The advantage is that we save time (not having to sort in preparation
for building the radix trees), and space in the generated bitcode files.
We could also do so for the callsite info records, but those are smaller
and less expensive to prepare.
This patch adds support for LLVM IR atomicrmw `fmaximum` and `fminimum`
instructions.
These mirror the `llvm.maximum.*` and `llvm.minimum.*` instructions, but
are atomic and use IEEE754 2019 handling for NaNs, which is different to
`fmax` and `fmin`. See:
https://llvm.org/docs/LangRef.html#llvm-minimum-intrinsic
for more details.
Future changes will allow this LLVM IR to be lowered to specialised
assembler instructions on suitable targets, such as AArch64.
We currently accept label arguments to inline asm calls. This support
predates both blockaddresses and callbr and is only covered by one X86
test. Remove it in favor of callbr (or at least blockaddress, though
that cannot guarantee correct codegen, just like using block labels
directly can't).
I didn't bother implementing bitcode upgrade support for this, but I can
add it if desired.
This patch adds support for LLVM IR atomicrmw `fmaximum` and `fminimum`
instructions.
These mirror the `llvm.maximum.*` and `llvm.minimum.*` instructions, but
are atomic and use IEEE754 2019 handling for NaNs, which is different to
`fmax` and `fmin`. See:
https://llvm.org/docs/LangRef.html#llvm-minimum-intrinsic
for more details.
Future changes will allow this LLVM IR to be lowered to specialised
assembler instructions on suitable targets, such as AArch64.
During the transition from debug intrinsics to debug records, we used
several different command line options to customise handling: the
printing of debug records to bitcode and textual could be independent of
how the debug-info was represented inside a module, whether the
autoupgrader ran could be customised. This was all valuable during
development, but now that totally removing debug intrinsics is coming
up, this patch removes those options in favour of a single flag
(experimental-debuginfo-iterators), which enables autoupgrade, in-memory
debug records, and debug record printing to bitcode and textual IR.
We need to do this ahead of removing the
experimental-debuginfo-iterators flag, to reduce the amount of
test-juggling that happens at that time.
There are quite a number of weird test behaviours related to this --
some of which I simply delete in this commit. Things like
print-non-instruction-debug-info.ll , the test suite now checks for
debug records in all tests, and we don't want to check we can print as
intrinsics. Or the update_test_checks tests -- these are duplicated with
write-experimental-debuginfo=false to ensure file writing for intrinsics
is correct, but that's something we're imminently going to delete.
A short survey of curious test changes:
* free-intrinsics.ll: we don't need to test that debug-info is a zero
cost intrinsic, because we won't be using intrinsics in the future.
* undef-dbg-val.ll: apparently we pinned this to non-RemoveDIs in-memory
mode while we sorted something out; it works now either way.
* salvage-cast-debug-info.ll: was testing intrinsics-in-memory get
salvaged, isn't necessary now
* localize-constexpr-debuginfo.ll: was producing "dead metadata"
intrinsics for optimised-out variable values, dbg-records takes the
(correct) representation of poison/undef as an operand. Looks like we
didn't update this in the past to avoid spurious test differences.
* Transforms/Scalarizer/dbginfo.ll: this test was explicitly testing
that debug-info affected codegen, and we deferred updating the tests
until now. This is just one of those silent gnochange issues that get
fixed by RemoveDIs.
Finally: I've added a bitcode test, dbg-intrinsics-autoupgrade.ll.bc,
that checks we can autoupgrade debug intrinsics that are in bitcode into
the new debug records.
This adds DWARF generation for fixed-point types. This feature is needed
by Ada.
Note that a pre-existing GNU extension is used in one case. This has
been emitted by GCC for years, and is needed because standard DWARF is
otherwise incapable of representing these types.
In Ada, an array can be packed and the elements can take less space than
their natural object size. For example, for this type:
type Packed_Array is array (4 .. 8) of Boolean;
pragma pack (Packed_Array);
... each element of the array occupies a single bit, even though the
"natural" size for a Boolean in memory is a byte.
In DWARF, this is represented by putting a DW_AT_bit_stride onto the
array type itself.
This patch adds a bit stride to DICompositeType so that gnat-llvm can
emit DWARF for these sorts of arrays.
Before the patch,
`writeCombinedGlobalValueSummary` traversed entire
`cfiFunction*` for each module, just to pick a few
symbols from `DefOrUseGUIDs`.
Now we change internals of `cfiFunctionDefs` and
`cfiFunctionDecls` to maintain a map from GUID to StringSet.
So now we iterate `DefOrUseGUIDs`, usually small,
and pick exact subset of symbols.
Sorting is not strictly necessary, but it
preserves the order of emitted values.
The module currently stores the target triple as a string. This means
that any code that wants to actually use the triple first has to
instantiate a Triple, which is somewhat expensive. The change in #121652
caused a moderate compile-time regression due to this. While it would be
easy enough to work around, I think that architecturally, it makes more
sense to store the parsed Triple in the module, so that it can always be
directly queried.
For this change, I've opted not to add any magic conversions between
std::string and Triple for backwards-compatibilty purses, and instead
write out needed Triple()s or str()s explicitly. This is because I think
a decent number of them should be changed to work on Triple as well, to
avoid unnecessary conversions back and forth.
The only interesting part in this patch is that the default triple is
Triple("") instead of Triple() to preserve existing behavior. The former
defaults to using the ELF object format instead of unknown object
format. We should fix that as well.
An Ada program can have types that are subranges of other types. This
patch adds a new DIType node, DISubrangeType, to represent this concept.
I considered extending the existing DISubrange to do this, but as
DISubrange does not derive from DIType, that approach seemed more
disruptive.
A DISubrangeType can be used both as an ordinary type, but also as the
type of an array index. This is also important for Ada.
Ada subrange types can also be stored using a bias. Representing this in
the DWARF required the use of an extension. GCC has been emitting this
extension for years, so I've reused it here.
Model C/C++ `errno` macro by adding a corresponding `errno`
memory location kind to the IR. Preliminary work to separate
`errno` writes from other memory accesses, to the benefit of
alias analyses and optimization correctness.
Previous discussion: https://discourse.llvm.org/t/rfc-modelling-errno-memory-effects/82972.
When creating `EnumDecl`s from DWARF for Objective-C `NS_ENUM`s, the
Swift compiler tries to figure out if it should perform "swiftification"
of that enum (which involves renaming the enumerator cases, etc.). The
heuristics by which it determines whether we want to swiftify an enum is
by checking the `enum_extensibility` attribute (because that's what
`NS_ENUM` pretty much are). Currently LLDB fails to attach the
`EnumExtensibilityAttr` to `EnumDecl`s it creates (because there's not
enough info in DWARF to derive it), which means we have to fall back to
re-building Swift modules on-the-fly, slowing down expression evaluation
substantially. This happens around
4b3931c8ce/lib/ClangImporter/ImportEnumInfo.cpp (L37-L59)
To speed up Swift exression evaluation, this patch proposes encoding the
C/C++/Objective-C `enum_extensibility` attribute in DWARF via a new
`DW_AT_APPLE_ENUM_KIND`. This would currently be only used from the LLDB
Swift plugin. But may be of interest to other language plugins as well
(though I haven't come up with a concrete use-case for it outside of
Swift).
I'm open to naming suggestions of the various new attributes/attribute
constants proposed here. I tried to be as generic as possible if we
wanted to extend it to other kinds of enum properties (e.g., flag
enums).
The new attribute would look as follows:
```
DW_TAG_enumeration_type
DW_AT_type (0x0000003a "unsigned int")
DW_AT_APPLE_enum_kind (DW_APPLE_ENUM_KIND_Closed)
DW_AT_name ("ClosedEnum")
DW_AT_byte_size (0x04)
DW_AT_decl_file ("enum.c")
DW_AT_decl_line (23)
DW_TAG_enumeration_type
DW_AT_type (0x0000003a "unsigned int")
DW_AT_APPLE_enum_kind (DW_APPLE_ENUM_KIND_Open)
DW_AT_name ("OpenEnum")
DW_AT_byte_size (0x04)
DW_AT_decl_file ("enum.c")
DW_AT_decl_line (27)
```
Absence of the attribute means the extensibility of the enum is unknown
and abides by whatever the language rules of that CU dictate.
This does feel like a big hammer for quite a specific use-case, so I'm
happy to discuss alternatives.
Alternatives considered:
* Re-using an existing DWARF attribute to express extensibility. E.g., a
`DW_TAG_enumeration_type` could have a `DW_AT_count` or
`DW_AT_upper_bound` indicating the number of enumerators, which could
imply closed-ness. I felt like a dedicated attribute (which could be
generalized further) seemed more applicable. But I'm open to re-using
existing attributes.
* Encoding the entire attribute string (i.e., `DW_TAG_LLVM_annotation
("enum_extensibility((open))")`) on the `DW_TAG_enumeration_type`. Then
in LLDB somehow parse that out into a `EnumExtensibilityAttr`. I haven't
found a great API in Clang to parse arbitrary strings into AST nodes
(the ones I've found required fully formed C++ constructs). Though if
someone knows of a good way to do this, happy to consider that too.
This PR removes the old `nocapture` attribute, replacing it with the new
`captures` attribute introduced in #116990. This change is
intended to be essentially NFC, replacing existing uses of `nocapture`
with `captures(none)` without adding any new analysis capabilities.
Making use of non-`none` values is left for a followup.
Some notes:
* `nocapture` will be upgraded to `captures(none)` by the bitcode
reader.
* `nocapture` will also be upgraded by the textual IR reader. This is to
make it easier to use old IR files and somewhat reduce the test churn in
this PR.
* Helper APIs like `doesNotCapture()` will check for `captures(none)`.
* MLIR import will convert `captures(none)` into an `llvm.nocapture`
attribute. The representation in the LLVM IR dialect should be updated
separately.
The ThinLTO index bitcode writer uses a helper forEachSummary to manage
preparation and writing of summaries needed for each distributed index
file. For alias summaries, it invokes the provided callback for the
aliasee as well, as we at least need to produce a value id for the
alias's summary. However, all summary generation for the aliasee itself
should be skipped on calls when IsAliasee is true. We invoke the
callback again if that value's summary is to be written as well.
We were asserting in debug mode when invoking collectMemProfCallStacks,
because a given stack id index was not in the StackIdIndicesToIndex
map. It was not added because the forEachSummary invocation that records
these ids in the map (invoked from the IndexBitcodeWriter constructor)
was correctly skipping this handling when invoked for aliasees. We need
the same guard in the invocation that calls collectMemProfCallStacks.
Note that this doesn't cause any real problems in a non-asserts build
as the missing map lookup will return the default 0 value from the map,
which isn't used since we don't actually write the corresponding
summary.
This introduces the `captures` attribute as described in:
https://discourse.llvm.org/t/rfc-improvements-to-capture-tracking/81420
This initial patch only introduces the IR/bitcode support for the
attribute and its in-memory representation as `CaptureInfo`. This will
be followed by a patch to upgrade and remove the `nocapture` attribute,
and then by actual inference/analysis support.
Based on the RFC feedback, I've used a syntax similar to the `memory`
attribute, though the only "location" that can be specified is `ret`.
I've added some pretty extensive documentation to LangRef on the
semantics. One non-obvious bit here is that using ptrtoint will not
result in a "return-only" capture, even if the ptrtoint result is only
used in the return value. Without this requirement we wouldn't be able
to continue ordinary capture analysis on the return value.
This patch introduces the LLVM components of a type sanitizer: a
sanitizer for type-based aliasing violations.
It is based on Hal Finkel's https://reviews.llvm.org/D32198.
C/C++ have type-based aliasing rules, and LLVM's optimizer can exploit
these given TBAA metadata added by Clang. Roughly, a pointer of given
type cannot be used to access an object of a different type (with, of
course, certain exceptions). Unfortunately, there's a lot of code in the
wild that violates these rules (e.g. for type punning), and such code
often must be built with -fno-strict-aliasing. Performance is often
sacrificed as a result. Part of the problem is the difficulty of finding
TBAA violations. Hopefully, this sanitizer will help.
For each TBAA type-access descriptor, encoded in LLVM's IR using
metadata, the corresponding instrumentation pass generates descriptor
tables. Thus, for each type (and access descriptor), we have a unique
pointer representation. Excepting anonymous-namespace types, these
tables are comdat, so the pointer values should be unique across the
program. The descriptors refer to other descriptors to form a type
aliasing tree (just like LLVM's TBAA metadata does). The instrumentation
handles the "fast path" (where the types match exactly and no
partial-overlaps are detected), and defers to the runtime to handle all
of the more-complicated cases. The runtime, of course, is also
responsible for reporting errors when those are detected.
The runtime uses essentially the same shadow memory region as tsan, and
we use 8 bytes of shadow memory, the size of the pointer to the type
descriptor, for every byte of accessed data in the program. The value 0
is used to represent an unknown type. The value -1 is used to represent
an interior byte (a byte that is part of a type, but not the first
byte). The instrumentation first checks for an exact match between the
type of the current access and the type for that address recorded in the
shadow memory. If it matches, it then checks the shadow for the
remainder of the bytes in the type to make sure that they're all -1. If
not, we call the runtime. If the exact match fails, we next check if the
value is 0 (i.e. unknown). If it is, then we check the shadow for the
remainder of the byes in the type (to make sure they're all 0). If
they're not, we call the runtime. We then set the shadow for the access
address and set the shadow for the remaining bytes in the type to -1
(i.e. marking them as interior bytes). If the type indicated by the
shadow memory for the access address is neither an exact match nor 0, we
call the runtime.
The instrumentation pass inserts calls to the memset intrinsic to set
the memory updated by memset, memcpy, and memmove, as well as
allocas/byval (and for lifetime.start/end) to reset the shadow memory to
reflect that the type is now unknown. The runtime intercepts memset,
memcpy, etc. to perform the same function for the library calls.
The runtime essentially repeats these checks, but uses the full TBAA
algorithm, just as the compiler does, to determine when two types are
permitted to alias. In a situation where access overlap has occurred and
aliasing is not permitted, an error is generated.
Clang's TBAA representation currently has a problem representing unions,
as demonstrated by the one XFAIL'd test in the runtime patch. We'll
update the TBAA representation to fix this, and at the same time, update
the sanitizer.
When the sanitizer is active, we disable actually using the TBAA
metadata for AA. This way we're less likely to use TBAA to remove memory
accesses that we'd like to verify.
As a note, this implementation does not use the compressed shadow-memory
scheme discussed previously
(http://lists.llvm.org/pipermail/llvm-dev/2017-April/111766.html). That
scheme would not handle the struct-path (i.e. structure offset)
information that our TBAA represents. I expect we'll want to further
work on compressing the shadow-memory representation, but I think it
makes sense to do that as follow-up work.
It goes together with the corresponding clang changes
(https://github.com/llvm/llvm-project/pull/76260) and compiler-rt
changes (https://github.com/llvm/llvm-project/pull/76261)
PR: https://github.com/llvm/llvm-project/pull/76259
When `-import-declaration` option is enabled, declaration import is
supported for functions. https://github.com/llvm/llvm-project/pull/88024
has the context for this option.
This patch supports declaration import for global variables in
distributed ThinLTO. The motivating use case is to propagate `dso_local`
attribute of global variables across modules, to optimize global
variable access when a binary is built with
`-fno-direct-access-external-data`.
* With `-fdirect-access-external-data`, non thread-local global
variables will [have `dso_local`
attributes](fe3c23b439/clang/lib/CodeGen/CodeGenModule.cpp (L1730-L1746)).
This optimizes the global variable access as shown by
https://gcc.godbolt.org/z/vMzWcKdh3
CallStackRadixTreeBuilder::build takes the parameter
MemProfFrameIndexes by value, involving copies:
std::optional<const llvm::DenseMap<FrameIdTy, LinearFrameId>>
MemProfFrameIndexes
Then "build" makes another copy of MemProfFrameIndexe and passes it to
encodeCallStack for every call stack, which is painfully slow.
This patch changes the type to a pointer so that we don't have to make
a copy every time we pass the argument.
Without this patch, it takes 553 seconds to run "llvm-profdata merge"
on a large MemProf raw profile. This patch shortenes that down to 67
seconds.
This reverts commit fdb050a5024320ec29d2edf3f2bc686c3a84abaa, and
restores ccb4702038900d82d1041ff610788740f5cef723, with a fix for build
bot failures.
Specifically, add ProfileData to the dependences of the BitWriter
library, which was causing shared library builds of LLVM to fail.
Reproduced the failure with a shared library build and confirmed this
change fixes that build failure.
Leverage the support added to represent allocation contexts in a more
compact way via a radix tree in the indexed profile to similarly reduce
sizes of the bitcode summaries.
For a large target, this reduced the size of the per-module summaries by
about 18% and in the distributed combined index files by 28%.
Currently, both
[TypeIdMap](67a1fdb014/llvm/include/llvm/IR/ModuleSummaryIndex.h (L1356))
and
[TypeIdCompatibleVtableMap](67a1fdb014/llvm/include/llvm/IR/ModuleSummaryIndex.h (L1363))
keep type-id as `std::string` in the combined index for LTO indexing
analysis.
With this change, index uses a unique-string-saver to own the string
copies and two maps above can use string references to save some memory.
This shows a 3% memory reduction (from 8.2GiB to 7.9GiB) in an internal
binary with high indexing memory usage.
The stack ids are hashes that are close to 64 bits in size, so emitting
as a pair of 32-bit fixed-width values is more efficient than a VBR.
This reduced the summary bitcode size for a large target by about 1%.
Bump the index version and ensure we can read the old format.
Improve the information printed when -memprof-report-hinted-sizes is
enabled. Now print the full context hash computed from the original
profile, similar to what we do when reporting matching statistics. This
will make it easier to correlate with the profile.
Note that the full context hash must be computed at profile match time
and saved in the metadata and summary, because we may trim the context
during matching when it isn't needed for distinguishing hotness.
Similarly, due to the context trimming, we may have more than one full
context id and total size pair per MIB in the metadata and summary,
which now get a list of these pairs.
Remove the old aggregate size from the metadata and summary support.
One other change from the prior support is that we no longer write the
size information into the combined index for the LTO backends, which
don't use this information, which reduces unnecessary bloat in
distributed index files.
Add a specification attribute to LLVM DebugInfo, which is analogous
to DWARF's DW_AT_specification. According to the DWARF spec:
"A debugging information entry that represents a declaration that
completes another (earlier) non-defining declaration may have a
DW_AT_specification attribute whose value is a reference to the
debugging information entry representing the non-defining declaration."
This patch allows types to be specifications of other types. This is
used by Swift to represent generic types. For example, given this Swift
program:
```
struct MyStruct<T> {
let t: T
}
let variable = MyStruct<Int>(t: 43)
```
The Swift compiler emits (roughly) an unsubtituted type for MyStruct<T>:
```
DW_TAG_structure_type
DW_AT_name ("MyStruct")
// "$s1w8MyStructVyxGD" is a Swift mangled name roughly equivalent to
// MyStruct<T>
DW_AT_linkage_name ("$s1w8MyStructVyxGD")
// other attributes here
```
And a specification for MyStruct<Int>:
```
DW_TAG_structure_type
DW_AT_specification (<link to "MyStruct">)
// "$s1w8MyStructVySiGD" is a Swift mangled name equivalent to
// MyStruct<Int>
DW_AT_linkage_name ("$s1w8MyStructVySiGD")
DW_AT_byte_size (0x08)
// other attributes here
```
An extra inhabitant is a bit pattern that does not represent a valid
value for instances of a given type. The number of extra inhabitants is
the number of those bit configurations.
This is used by Swift to save space when composing types. For example,
because Bool only needs 2 bit patterns to represent all of its values
(true and false), an Optional<Bool> only occupies 1 byte in memory by
using a bit configuration that is unused by Bool. Which bit patterns are
unused are part of the ABI of the language.
Since Swift generics are not monomorphized, by using dynamic libraries
you can have generic types whose size, alignment, etc, are known only
at runtime (which is why this feature is needed).
This patch adds num_extra_inhabitants to LLVM-IR debug info and in DWARF
as an Apple extension.
# What
This PR renames the newly-introduced llvm attribute
`sanitize_realtime_unsafe` to `sanitize_realtime_blocking`. Likewise,
sibling variables such as `SanitizeRealtimeUnsafe` are renamed to
`SanitizeRealtimeBlocking` respectively. There are no other functional
changes.
# Why?
- There are a number of problems that can cause a function to be
real-time "unsafe",
- we wish to communicate what problems rtsan detects and *why* they're
unsafe, and
- a generic "unsafe" attribute is, in our opinion, too broad a net -
which may lead to future implementations that need extra contextual
information passed through them in order to communicate meaningful
reasons to users.
- We want to avoid this situation and make the runtime library boundary
API/ABI as simple as possible, and
- we believe that restricting the scope of attributes to names like
`sanitize_realtime_blocking` is an effective means of doing so.
We also feel that the symmetry between `[[clang::blocking]]` and
`sanitize_realtime_blocking` is easier to follow as a developer.
# Concerns
- I'm aware that the LLVM attribute `sanitize_realtime_unsafe` has been
part of the tree for a few weeks now (introduced here:
https://github.com/llvm/llvm-project/pull/106754). Given that it hasn't
been released in version 20 yet, am I correct in considering this to not
be a breaking change?
A call to a function that has this attribute is not a source of
divergence, as used by UniformityAnalysis. That allows a front-end to
use known-name calls as an instruction extension mechanism (e.g.
https://github.com/GPUOpen-Drivers/llvm-dialects ) without such a call
being a source of divergence.
