[llvm-dev] Any way to disable the alignment of functions?

Rahman Lavaee Mashhadi via llvm-dev llvm-dev at lists.llvm.org
Sun Dec 27 14:35:42 PST 2015


It seems that adding the “OptimizeForSize” attribute to every function causes that function to be aligned with the minimum alignment required by the target.

> On Dec 27, 2015, at 4:23 PM, Rahman Lavaee Mashhadi <rlavaee at icloud.com> wrote:
> Hi,
> I am trying to test if disabling function (entry) alignment has any effect on performance. On my x86 machine, all functions seem to get aligned to 16-bytes.
> I know that GCC provides a flag (-fno-align-functions) to disable function alignment. However, I could not find the similar flag for clang. 
> According to the following quote from LLVM doc, clang delegates the job to the target in case no attributes are specified for a function.
> "An explicit alignment may be specified for a function. If not present, or if the alignment is set to zero, the alignment of the function is set by the target to whatever it feels convenient.”
> So, as a first try, I dived into the “EmitAlignment” code in AsmPrinter.cpp and simply removed the function alignment part. Interestingly the compilation seems flawless at many cases (large programs) but on a few programs, it produces a segfault-generating executable. So my guess is that for some functions, alignment is necessary for soundness, not just performance.
> regards

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