Non-optimization of std::min({a,b,c})

Harris__John · ‎12-13-2018

See This example on CompilerExplorer

Function foo() and bar() generate the same few assembly instructions with GCC or Clang, but with ICPC (19.0.0),
the function baz() is much larger and more complex.  In particular, it has loops.

icpc -O2 -std=c++17 -Wall -Werror

#include <algorithm>

int foo(int a, int b, int c) {
    return std::min(std::min(a,b), c); // this is correctly optimized
}

int bar(int a, int b, int c) {
    return std::min({a,b,c}); // this should be optmized down to what's in foo()
}

int main(int, char **)
{
    return foo(1,2,3);
}

foo(int, int, int):
        cmp       esi, edi                                      #4.12
        cmovl     edi, esi                                      #4.12
        cmp       edi, edx                                      #4.12
        cmovl     edx, edi                                      #4.12
        mov       eax, edx                                      #4.12
        ret                                                     #4.12
bar(int, int, int):
        mov       DWORD PTR [-24+rsp], edi                      #8.22
        lea       rcx, QWORD PTR [-12+rsp]                      #79.37
        mov       DWORD PTR [-8+rcx], esi                       #8.24
        lea       rax, QWORD PTR [-24+rsp]                      #5619.14
        mov       DWORD PTR [8+rax], edx                        #8.26
        cmp       rax, rcx                                      #5591.22
        je        ..B3.7        # Prob 5%                       #5591.22
        lea       rdx, QWORD PTR [-20+rsp]                      #5588.5
        cmp       rdx, rcx                                      #5594.27
        je        ..B3.7        # Prob 50%                      #5594.27
        xor       edx, edx                                      #5594.7
        jmp       ..B3.9        # Prob 100%                     #5594.7
..B3.4:                         # Preds ..B3.12
        cmp       rcx, 2                                        #5594.7
        jae       ..B3.7        # Prob 1%                       #5594.7
        lea       rcx, QWORD PTR [-20+rsp+rdx*8]                #43.17
        mov       edx, DWORD PTR [rcx]                          #43.17
        cmp       edx, DWORD PTR [rax]                          #5596.4
        cmovl     rax, rcx                                      #5596.4
..B3.7:                         # Preds ..B3.1 ..B3.2 ..B3.4 ..B3.5
        mov       eax, DWORD PTR [rax]                          #8.12
        ret                                                     #8.12
..B3.9:                         # Preds ..B3.3 ..B3.11
        mov       ecx, DWORD PTR [rax]                          #43.26
        cmp       ecx, DWORD PTR [-20+rsp+rdx*8]                #43.26
        jle       ..B3.11       # Prob 50%                      #43.26
        lea       rax, QWORD PTR [-20+rsp+rdx*8]                #5594.16
        mov       ecx, DWORD PTR [rax]                          #43.26
..B3.11:                        # Preds ..B3.10 ..B3.9
        cmp       ecx, DWORD PTR [-16+rsp+rdx*8]                #5596.4
        lea       rsi, QWORD PTR [-16+rsp+rdx*8]                #5594.16
        cmovg     rax, rsi                                      #5596.4
        inc       rdx                                           #5594.7
        cmp       rdx, 1                                        #5594.7
        jb        ..B3.9        # Prob 64%                      #5594.7
        lea       rcx, QWORD PTR [rdx+rdx]                      #5595.2
        jmp       ..B3.4        # Prob 100%                     #5595.2

Viet_H_Intel · ‎12-14-2018

Thanks! I've reported this issue to our developer. Case number is CMPLRIL0-30924.

Regards,

Viet