[llvm-bugs] [Bug 43003] New: Use derefenceable info to replace branch with select

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43003

            Bug ID: 43003
           Summary: Use derefenceable info to replace branch with select
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: llvm-bugs at lists.llvm.org

void foo(int arr[static 1024]) {
    for (int i = 0; i < 1024; ++i) {
        if (arr[i] < 0)
            arr[i] = 0;
    }
}

void foo2(int arr[static 1024]) {
    for (int i = 0; i < 1024; ++i) {
        arr[i] = arr[i] < 0 ? 0 : arr[i];
    }
}

void foo3(int arr[1024]) {
    for (int i = 0; i < 1024; ++i) {
        arr[i] = arr[i] < 0 ? 0 : arr[i];
    }
}

void foo4(int arr[1024]) {
    for (int i = 0; i < 1024; ++i) {
         if (arr[i] < 0)
            arr[i] = 0;
    }
}

Replace:
if (arr[i] < 0)
            arr[i] = 0;

with: arr[i] = arr[i] < 0 ? 0 : arr[i];

If we know that 'i' is always in dereferenceable range of 'arr'.
'dereferenceable' says that we can write to memory (it is not a read only
memory).

in 'foo', we know i32* nocapture dereferenceable(4096) %arr and we know i is in
range <0, 1024), so we should be able to replace branch with a elect.

I am not sure about 'foo4' case,  we can do nothing I think since 'arr' could
be read only.


Motivation? Clang -O3 -mavx2

Replace:

foo:                                    # @foo
        xor     eax, eax
        vpxor   xmm0, xmm0, xmm0
.LBB0_1:                                # =>This Inner Loop Header: Depth=1
        vmovdqu ymm1, ymmword ptr [rdi + rax]
        vmovdqu ymm2, ymmword ptr [rdi + rax + 32]
        vmovdqu ymm3, ymmword ptr [rdi + rax + 64]
        vmovdqu ymm4, ymmword ptr [rdi + rax + 96]
        vpmaskmovd      ymmword ptr [rdi + rax], ymm1, ymm0
        vpmaskmovd      ymmword ptr [rdi + rax + 32], ymm2, ymm0
        vpmaskmovd      ymmword ptr [rdi + rax + 64], ymm3, ymm0
        vpmaskmovd      ymmword ptr [rdi + rax + 96], ymm4, ymm0
        vmovdqu ymm1, ymmword ptr [rdi + rax + 128]
        vmovdqu ymm2, ymmword ptr [rdi + rax + 160]
        vmovdqu ymm3, ymmword ptr [rdi + rax + 192]
        vmovdqu ymm4, ymmword ptr [rdi + rax + 224]
        vpmaskmovd      ymmword ptr [rdi + rax + 128], ymm1, ymm0
        vpmaskmovd      ymmword ptr [rdi + rax + 160], ymm2, ymm0
        vpmaskmovd      ymmword ptr [rdi + rax + 192], ymm3, ymm0
        vpmaskmovd      ymmword ptr [rdi + rax + 224], ymm4, ymm0
        add     rax, 256
        cmp     rax, 4096
        jne     .LBB0_1
        vzeroupper
        ret

With a really nice code:
foo2:                                   # @foo2
        xor     eax, eax
        vpxor   xmm0, xmm0, xmm0
.LBB1_1:                                # =>This Inner Loop Header: Depth=1
        vpmaxsd ymm1, ymm0, ymmword ptr [rdi + 4*rax]
        vpmaxsd ymm2, ymm0, ymmword ptr [rdi + 4*rax + 32]
        vpmaxsd ymm3, ymm0, ymmword ptr [rdi + 4*rax + 64]
        vpmaxsd ymm4, ymm0, ymmword ptr [rdi + 4*rax + 96]
        vmovdqu ymmword ptr [rdi + 4*rax], ymm1
        vmovdqu ymmword ptr [rdi + 4*rax + 32], ymm2
        vmovdqu ymmword ptr [rdi + 4*rax + 64], ymm3
        vmovdqu ymmword ptr [rdi + 4*rax + 96], ymm4
        add     rax, 32
        cmp     rax, 1024
        jne     .LBB1_1
        vzeroupper
        ret


I believe simple case like:
void bar(int arr[static 1024]) {
    if (arr[80] < 0)
        arr[80] = 0;
}

could handle SimplifyCFG and turn branch to select.

Suprisingly:
void bar(int *arr) {
    if (arr[80] < 7)
        arr[80] = 7;
}

ICC generates cmovge.. ICC ignores the fact that 'arr' could be read only?

The motivation loop case I dont know.. maybe IndVars?

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