[PATCH] D110867: X86InstrInfo: Support immediates that are +1/-1 different in optimizeCompareInstr

[Matthias Braun via Phabricator via llvm-commits]

MatzeB added inline comments.


================
Comment at: llvm/lib/Target/X86/X86InstrInfo.cpp:4495
+      case X86::COND_L: // x <s (C + 1)  -->  x <=s C
+        if (ImmDelta != 1 || APInt(BitWidth, INT64_MIN) == CmpValue)
+          return false;
----------------
foad wrote:
> Surely you need `APInt::getSignedMinValue(BitWidth)`?
Good point. Changed them now.

I just realized that technically this check isn't necessary as the tests I added still pass. This is because the `ImmDelta` calculations are actually done on `int64_t` values so they don't wrap around for 8/16/32 bit immediates and instead just overflow to an unrepresentable immediate. We would have overflow problems for 64bit immediates, but x86 doesn't support those.

Anyway I think I'll keep the tests here anyway as I like the expressed intent, and if someone ever introduces a 64bit immediates we are prepared :)


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D110867/new/

https://reviews.llvm.org/D110867