duplicated compare instruction #8733

llvmbot · 2010-10-12T15:22:36Z


Bugzilla Link	8361
Resolution	FIXED
Resolved on	May 05, 2011 12:04
Version	trunk
OS	Linux
Blocks	#8497
Reporter	LLVM Bugzilla Contributor
CC	@atrick,@lattner,@efriedma-quic,@ggreif

Extended Description

Keywords: duplicate duplicated redundant repeated compare comparison cmp

The compare instruction is repeated unnecessarily in the code generated from this:

const char *f(int x, int y)
{
return x < y ? "<" : x == y ? "=" : ">";
}

I've seen this with the ARM and x86 back ends.

Currently on r116298.

lattner · 2010-10-12T18:20:44Z

Yep, this is because MachineCSE isn't doing everything it needs to

ggreif · 2010-10-12T23:31:05Z

This problem shows up on all targets I look at:

PPC32:
f: # @f

    cmpw 0, 3, 4
    blt 0, .LBB0_4

    cmplw 0, 3, 4
    bne 0, .LBB0_3

Thumb:
f:
@ BB#0: @ %entry
cmp r0, r1
blt .LBB0_4
@ BB#1: @ %cond.false
cmp r0, r1
bne .LBB0_3

ARM:
f: @ @f
@ BB#0: @ %entry
cmp r0, r1
ldrlt r0, .LCPI0_2
bxlt lr
ldr r3, .LCPI0_1
cmp r0, r1
mov r0, r3

x86:
f: # @f

    movl    8(%esp), %eax
    movl    4(%esp), %ecx
    cmpl    %eax, %ecx
    jl      .LBB0_2

    cmpl    %eax, %ecx
    movl    $.L.str1, %ecx

Starting investigation, maybe I can come up with something acceptable.

efriedma-quic · 2011-05-05T19:04:32Z

r130928.

llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 3, 2021

This issue was closed.