I found this bug when performing a two-stage build of clang with
Function Specialization enabled and tuned aggressively. The crash
appears only on release builds.
Fixes https://github.com/llvm/llvm-project/issues/55000.
Before accessing the contents of the ArgInfo iterator inside
SCCPInstVisitor::markArgInFuncSpecialization, we should be
checking that the iterator is valid.
Differential Revision: https://reviews.llvm.org/D124114
The current implementation of Function Specialization does not allow
specializing more than one arguments per function call, which is a
limitation I am lifting with this patch.
My main challenge was to choose the most suitable ADT for storing the
specializations. We need an associative container for binding all the
actual arguments of a specialization to the function call. We also
need a consistent iteration order across executions. Lastly we want
to be able to sort the entries by Gain and reject the least profitable
ones.
MapVector fits the bill but not quite; erasing elements is expensive
and using stable_sort messes up the indices to the underlying vector.
I am therefore using the underlying vector directly after calculating
the Gain.
Differential Revision: https://reviews.llvm.org/D119880
`ArgInfo` is reduced to only contain a pair of {formal,actual} values.
The specialized function `Fn` and the `Partial` flag are redundant in
this structure. The `Gain` is moved to a new struct `SpecializationInfo`.
The value mappings created by cloneCandidateFunction() are being used
by rewriteCallSites() for matching the formal arguments of recursive
functions.
The list of specializations is passed by reference to calculateGains()
instead of being returned by value.
The `IsPartial` flag is removed from isArgumentInteresting() and
getPossibleConstants() as it's no longer used anywhere in the code.
Differential Revision: https://reviews.llvm.org/D120753
Extended value is known to be inside range smaller than full one.
Prevent SCCP to mark such value as overdefined.
Fixes PR52253
Differential Revision: https://reviews.llvm.org/D112721
In SCCPSolver::markArgInFuncSpecialization, the ValueState map may be
reallocated *after* the initial ValueLatticeElement reference is grabbed, but
*before* its use in copy initialization. This causes a use-after-free. To fix
this, this commit changes the behavior to create the new ValueLatticeElement
before assigning the old one to it.
Patch by: https://github.com/duck-37/
Differential Revision: https://reviews.llvm.org/D111112
This adds a function specialization pass to LLVM. Constant parameters
like function pointers and constant globals are propagated to the callee by
specializing the function.
This is a first version with a number of limitations:
- The pass is off by default, so needs to be enabled on the command line,
- It does not handle specialization of recursive functions,
- It does not yet handle constants and constant ranges,
- Only 1 argument per function is specialised,
- The cost-model could be further looked into, and perhaps related,
- We are not yet caching analysis results.
This is based on earlier work by Matthew Simpson (D36432) and Vinay Madhusudan.
More recently this was also discussed on the list, see:
https://lists.llvm.org/pipermail/llvm-dev/2021-March/149380.html.
The motivation for this work is that function specialisation often comes up as
a reason for performance differences of generated code between LLVM and GCC,
which has this enabled by default from optimisation level -O3 and up. And while
this certainly helps a few cpu benchmark cases, this also triggers in real
world codes and is thus a generally useful transformation to have in LLVM.
Function specialisation has great potential to increase compile-times and
code-size. The summary from some investigations with this patch is:
- Compile-time increases for short compile jobs is high relatively, but the
increase in absolute numbers still low.
- For longer compile-jobs, the extra compile time is around 1%, and very much
in line with GCC.
- It is difficult to blame one thing for compile-time increases: it looks like
everywhere a little bit more time is spent processing more functions and
instructions.
- But the function specialisation pass itself is not very expensive; it doesn't
show up very high in the profile of the optimisation passes.
The goal of this work is to reach parity with GCC which means that eventually
we would like to get this enabled by default. But first we would like to address
some of the limitations before that.
Differential Revision: https://reviews.llvm.org/D93838
This refactors SCCP and creates a SCCPSolver interface and class so that it can
be used by other passes and transformations. We will use this in D93838, which
adds a function specialisation pass.
This is based on an early version by Vinay Madhusudan.
Differential Revision: https://reviews.llvm.org/D93762
