[PATCH] D110339: SelectionDAGBuilder: Improve canonicalization by not swapping branch targets

[Craig Topper via Phabricator via llvm-commits]

craig.topper added inline comments.


================
Comment at: llvm/test/CodeGen/X86/2006-08-21-ExtraMovInst.ll:15
+; CHECK-NEXT:    cmpl $39, %ecx
+; CHECK-NEXT:    jle .LBB0_1
 ; CHECK-NEXT:  # %bb.2: # %bb12
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MatzeB wrote:
> RKSimon wrote:
> > This kind of thing is going to increase EFLAGS usage - I addressed this for CMOVs + SETs with SGT/UGT in D101074 but haven't had time to address JMP/BRANCH, or add the SLE/ULE equivalent yet.
> > This kind of thing is going to increase EFLAGS usage
> 
> Wow, I was not aware that affects performance. Does that mean MachineCodePlacement / analyzeBranch can pessimize code when it flips the true/false branches and negates the condition of a jump?
At least on Intel CPUs, I'm not aware of a performance issue for jumps. There is an extra uop for CMOV and SETCC conditions that use the C flag and Z flag. That would cmovbe, cmova, setbe, and seta. There is no extra uop for branches on the same conditions.

I can't speak to AMD CPUs.


Repository:
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CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D110339/new/

https://reviews.llvm.org/D110339