[llvm-bugs] [Bug 52032] New: Suboptimal materialization of += 1 as -= -1

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Fri Oct 1 10:54:24 PDT 2021


            Bug ID: 52032
           Summary: Suboptimal materialization of += 1 as -= -1
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, pengfei.wang at intel.com,
                    spatel+llvm at rotateright.com

struct b 
  int a[100000];

plus1(struct b *a)
  int i;
  for (i=0;i<64;i++)

Trunk -O3 -march=haswell:

plus1(b*): # @plus1(b*)
  vpcmpeqd %ymm0, %ymm0, %ymm0
  vmovdqu (%rdi), %ymm1
  vmovdqu 32(%rdi), %ymm2
  vmovdqu 64(%rdi), %ymm3
  vmovdqu 96(%rdi), %ymm4
  vpsubd %ymm0, %ymm1, %ymm1
  vmovdqu %ymm1, (%rdi)
  vpsubd %ymm0, %ymm2, %ymm1
  vmovdqu %ymm1, 32(%rdi)
  vpsubd %ymm0, %ymm3, %ymm1
  vmovdqu %ymm1, 64(%rdi)
  vpsubd %ymm0, %ymm4, %ymm1
  vmovdqu %ymm1, 96(%rdi)
  vmovdqu 128(%rdi), %ymm1
  vpsubd %ymm0, %ymm1, %ymm1
  vmovdqu %ymm1, 128(%rdi)
  vmovdqu 160(%rdi), %ymm1
  vpsubd %ymm0, %ymm1, %ymm1
  vmovdqu %ymm1, 160(%rdi)
  vmovdqu 192(%rdi), %ymm1
  vpsubd %ymm0, %ymm1, %ymm1
  vmovdqu %ymm1, 192(%rdi)
  vmovdqu 224(%rdi), %ymm1
  vpsubd %ymm0, %ymm1, %ymm0
  vmovdqu %ymm0, 224(%rdi)

ICC/GCC produces:

  vmovdqu .L_2il0floatpacket.0(%rip), %ymm7 #12.14
  vpaddd (%rdi), %ymm7, %ymm0 #12.5
  vpaddd 32(%rdi), %ymm7, %ymm1 #12.5
  vpaddd 64(%rdi), %ymm7, %ymm2 #12.5
  vpaddd 96(%rdi), %ymm7, %ymm3 #12.5
  vpaddd 128(%rdi), %ymm7, %ymm4 #12.5
  vpaddd 160(%rdi), %ymm7, %ymm5 #12.5
  vpaddd 192(%rdi), %ymm7, %ymm6 #12.5
  vpaddd 224(%rdi), %ymm7, %ymm8 #12.5
  vmovdqu %ymm0, (%rdi) #12.5
  vmovdqu %ymm1, 32(%rdi) #12.5
  vmovdqu %ymm2, 64(%rdi) #12.5
  vmovdqu %ymm3, 96(%rdi) #12.5
  vmovdqu %ymm4, 128(%rdi) #12.5
  vmovdqu %ymm5, 160(%rdi) #12.5
  vmovdqu %ymm6, 192(%rdi) #12.5
  vmovdqu %ymm8, 224(%rdi) #12.5
  vzeroupper #13.1
  ret #13.1

LLVM could just use:
  .long 1 # 0x1
plus1(b*): # @plus1(b*)
  vbroadcastss .LCPI0_0(%rip), %ymm0 # ymm0 = [1,1,1,1,1,1,1,1]


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