When a pointer to a tracked alloca is passed to a call that may write
through it (e.g. foo(&x)), the callee can modify the variable's stack
home. The assignment tracking analysis didn't account for this, which
could cause the debugger to show stale values after such calls.
Consider:
```
int x = 1;
foo(&x); // might set x to 99
x = 2; // store deleted by DSE
```
Without this patch, the analysis still thinks the stack home holds
assignment `!id1` after the call. When it later sees the `dbg_assign`
for the deleted store, the mismatch causes it to fall back to the old
debug value (1) , which is wrong.
Fix this by detecting calls where a tracked `alloca` escapes as an
argument and treating them the same way we already treat untagged
stores, set both `StackHome` and `Debug` to NoneOrPhi (unknown
assignment) and keep `LocKind` as Mem (the stack slot is still the right
place to look). This causes a `DBG_VALUE` with `DW_OP_deref` to be
emitted after
the call, telling the debugger to read whatever is actually in memory.
Pointer arguments with readonly, readnone, or byval attributes are
skipped since the callee either cannot modify the original memory or
receives a copy. Intrinsics are also skipped since their memory effects
are already modeled individually (e.g. memset/memcpy as stores, lifetime
markers as no-ops).
---------
Co-authored-by: Shivam Kunwar <phyBrackets@users.noreply.github.com>
As we approach the state where support for debug intrinsics is dropping and
we print and use debug records by default, the documentation should be updated
to refer to debug records as the primary debug info representation, with
debug intrinsics being relegated to an optional alternative.
This patch performs a few updates:
- Replace references to intrinsics with references to records across all
the documentation.
- Replace intrinsics with records in code examples.
- Move debug records prior to debug intrinsics in the
SourceLevelDebugging document, and change text to refer to them as the
primary representation.
- Add release notes describing the change.
Remove LLVM flag -experimental-assignment-tracking. Assignment tracking is
still enabled from Clang with the command line -Xclang
-fexperimental-assignment-tracking which tells Clang to ask LLVM to run the
pass declare-to-assign. That pass converts conventional debug intrinsics to
assignment tracking metadata. With this patch it now also sets a module flag
debug-info-assignment-tracking with the value `i1 true` (using the flag conflict
rule `Max` since enabling assignment tracking on IR that contains only
conventional debug intrinsics should cause no issues).
Update the docs and tests too.
Reviewed By: CarlosAlbertoEnciso
Differential Revision: https://reviews.llvm.org/D142027
The Assignment Tracking debug-info feature is outlined in this RFC:
https://discourse.llvm.org/t/
rfc-assignment-tracking-a-better-way-of-specifying-variable-locations-in-ir
Add documentation outlining the intent and design.
