[PATCH] D79532: [SelectionDAG] Don't promote the alignment of allocas beyond the stack alignment.

[Eli Friedman via Phabricator via llvm-commits]

efriedma created this revision.
efriedma added reviewers: arsenm, craig.topper, sdesmalen.
Herald added subscribers: kerbowa, dmgreen, arphaman, hiraditya, nhaehnle, wdng, jvesely.
Herald added a reviewer: rengolin.
Herald added a project: LLVM.
efriedma added a subscriber: simon_tatham.

allocas in LLVM IR have a specified alignment.  When that alignment is specified, the alloca has at least that alignment at runtime.

If the specified type of the alloca has a higher preferred alignment, SelectionDAG currently ignores that specified alignment, and increases the alignment.  It does this even if it would trigger stack realignment. I don't think this makes sense, so this patch changes that.

I was looking into this for SVE in particular: for SVE, overaligning vscale'ed types is extra expensive because it requires realigning the stack multiple times, or using dynamic allocation. (This currently isn't implemented.)

I updated the expected assembly for a couple tests; in particular, for arg-copy-elide.ll, the optimization in question does not increase the alignment the way SelectionDAG normally would. For the rest, I just increased the specified alignment on the allocas to match what SelectionDAG was inferring.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D79532

Files:
  llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
  llvm/test/CodeGen/AArch64/sve-alloca-stackid.ll
  llvm/test/CodeGen/AMDGPU/private-element-size.ll
  llvm/test/CodeGen/AMDGPU/stack-pointer-offset-relative-frameindex.ll
  llvm/test/CodeGen/ARM/alloc-no-stack-realign.ll
  llvm/test/CodeGen/Thumb2/mve-basic.ll
  llvm/test/CodeGen/X86/arg-copy-elide.ll
  llvm/test/CodeGen/X86/avx2-vbroadcast.ll
  llvm/test/CodeGen/X86/avx512-intel-ocl.ll
  llvm/test/CodeGen/X86/movtopush.ll

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