[PATCH] D115969: [Support] improve known bits analysis for leading zeros of multiply

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: lebedev.ri, efriedma, RKSimon.
Herald added subscribers: dexonsmith, pengfei, hiraditya, mcrosier.
spatel requested review of this revision.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.

Instead of summing leading zeros on the input operands, multiply the max possible values of those inputs and count the leading zeros of the result. This can give us an extra zero bit (typically in cases where one of the operands is a known constant).

This allows folding away the remaining 'add' ops in the motivating bug (modeled in the PhaseOrdering IR test):
https://llvm.org/PR48399


https://reviews.llvm.org/D115969

Files:
  llvm/lib/Support/KnownBits.cpp
  llvm/test/CodeGen/X86/mul128.ll
  llvm/test/Transforms/InstCombine/icmp-mul.ll
  llvm/test/Transforms/InstCombine/narrow-switch.ll
  llvm/test/Transforms/PhaseOrdering/X86/pixel-splat.ll

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D115969.395213.patch
Type: text/x-patch
Size: 8655 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20211217/caa78228/attachment.bin>