x86 optimization: Missed merging comparison of isunordered() and islessgreater()

[Explorer09]

#include <limits.h>
#include <math.h>
#include <stdlib.h>
int func(double x, double y) {
    if (isunordered(x, y))
        return INT_MIN;
    if (islessgreater(x, y))
        return x > y ? 1 : -1;
    return 0;
}

https://godbolt.org/z/fPxT3qfPG

x86-64 clang 21.1.0 with -Os option produces:

func:
        ucomisd xmm0, xmm1
        jp      .LBB0_1
        ucomisd xmm0, xmm1
        je      .LBB0_3
        seta    al
        movzx   eax, al
        lea     eax, [2*rax - 1]
        ret
.LBB0_3:
        xor     eax, eax
        ret
.LBB0_1:
        mov     eax, -2147483648
        ret

Note the duplicate UCOMISD instructions.

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Kang-Che Sung (宋岡哲) (Explorer09)

```c #include <limits.h> #include <math.h> #include <stdlib.h> int func(double x, double y) { if (isunordered(x, y)) return INT_MIN; if (islessgreater(x, y)) return x > y ? 1 : -1; return 0; } ```

https://godbolt.org/z/fPxT3qfPG

x86-64 clang 21.1.0 with -Os option produces:

func:
        ucomisd xmm0, xmm1
        jp      .LBB0_1
        ucomisd xmm0, xmm1
        je      .LBB0_3
        seta    al
        movzx   eax, al
        lea     eax, [2*rax - 1]
        ret
.LBB0_3:
        xor     eax, eax
        ret
.LBB0_1:
        mov     eax, -2147483648
        ret

Note the duplicate UCOMISD instructions.

[Arghnews]

I'll have a look at this if that's ok, please assign to me, thanks

[Explorer09]

A minor update: Sometimes this problem can disappear if I alter the return 0; on the last line. For example by changing the return value to 100. The problem can also disappear if there is an empty inline asm between the conditional block and the return 0; statement.

int func_b(double x, double y) {
    if (isunordered(x, y))
        return INT_MIN;
    if (islessgreater(x, y))
        return x > y ? 1 : -1;
    __asm__("");
    return 0;
}

int func_c(double x, double y) {
    if (isunordered(x, y))
        return INT_MIN;
    if (islessgreater(x, y))
        return x > y ? 1 : -1;
    return 100;
}