[llvm-bugs] [Bug 31693] New: Failure to recognise some integer MIN/MAX CLAMP patterns

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=31693

            Bug ID: 31693
           Summary: Failure to recognise some integer MIN/MAX CLAMP
                    patterns
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: llvm-dev at redking.me.uk
                CC: filcab at gmail.com, llvm-bugs at lists.llvm.org,
                    spatel+llvm at rotateright.com
    Classification: Unclassified

Value clamping is often implemented using either of these patterns:

#define MIN(v,a) ((v) < (a) ? (v) : (a))
#define MAX(v,a) ((v) < (a) ? (a) : (v))
#define CLAMP(v,l,h) MIN(MAX((v),(l)),(h))

//#define CLAMP(v,l,h) ((v) < (l) ? (l) : ((v) > (h) ? (h) : (v)))

void clamp_v4u32(unsigned int *a) {
  for (int i = 0; i != 4; ++i, ++a) {
    unsigned int v = *a;
    v = CLAMP(v, 15, 255);
    *a = v;
  }
}

The first implementation nicely lowers to a pair of UMIN/UMAX instructions:

llc -mcpu=btver2 -mtriple=x86_64-unknown

define void @clamp_v4u32((i32* nocapture) {
  %2 = bitcast i32* %0 to <4 x i32>*
  %3 = load <4 x i32>, <4 x i32>* %2, align 4
  %4 = icmp ugt <4 x i32> %3, <i32 15, i32 15, i32 15, i32 15>
  %5 = select <4 x i1> %4, <4 x i32> %3, <4 x i32> <i32 15, i32 15, i32 15, i32
15>
  %6 = icmp ult <4 x i32> %5, <i32 255, i32 255, i32 255, i32 255>
  %7 = select <4 x i1> %6, <4 x i32> %5, <4 x i32> <i32 255, i32 255, i32 255,
i32 255>
  %8 = bitcast i32* %0 to <4 x i32>*
  store <4 x i32> %7, <4 x i32>* %8, align 4
  ret void
}

clamp_v4u32:
        vmovdqu (%rdi), %xmm0
        vpmaxud .LCPI0_0(%rip), %xmm0, %xmm0
        vpminud .LCPI0_1(%rip), %xmm0, %xmm0
        vmovdqu %xmm0, (%rdi)
        retq

The second struggles fails to recognise that we can safely combine the second
comparison under certain circumstances:

define void @clamp_v4u32(i32* nocapture) {
  %2 = bitcast i32* %0 to <4 x i32>*
  %3 = load <4 x i32>, <4 x i32>* %2, align 4
  %4 = icmp ult <4 x i32> %3, <i32 15, i32 15, i32 15, i32 15>
  %5 = icmp ult <4 x i32> %3, <i32 255, i32 255, i32 255, i32 255>
  %6 = select <4 x i1> %5, <4 x i32> %3, <4 x i32> <i32 255, i32 255, i32 255,
i32 255>
  %7 = select <4 x i1> %4, <4 x i32> <i32 15, i32 15, i32 15, i32 15>, <4 x
i32> %6
  %8 = bitcast i32* %0 to <4 x i32>*
  store <4 x i32> %7, <4 x i32>* %8, align 4
  ret void
}

clamp_v4u32:
        vmovdqu (%rdi), %xmm0
        vmovdqa .LCPI0_1(%rip), %xmm2   # xmm2 =
[2147483663,2147483663,2147483663,2147483663]
        vpxor   .LCPI0_0(%rip), %xmm0, %xmm1
        vpminud .LCPI0_3(%rip), %xmm0, %xmm0
        vpcmpgtd        %xmm1, %xmm2, %xmm1
        vblendvps       %xmm1, .LCPI0_2(%rip), %xmm0, %xmm0
        vmovups %xmm0, (%rdi)
        retq

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