[llvm-bugs] [Bug 39968] New: [x86-64] Suboptimal codegen for (!x) as opposed to (0 == x)

via llvm-bugs llvm-bugs at lists.llvm.org
Tue Dec 11 22:27:49 PST 2018


            Bug ID: 39968
           Summary: [x86-64] Suboptimal codegen for (!x) as opposed to (0
                    == x)
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: arthur.j.odwyer at gmail.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org


    #include <stdint.h>
    struct u128 { uint64_t x, y; };

    u128 shl2(uint64_t rdi, uint64_t rdx, int n)
        if (n & 64) rdx = rdi;
    #if WORSE
        if (!(n & 64)) rdi = 0;
        if (0 == (n & 64)) rdi = 0;
        return {rdi,rdx};

Without -DWORSE, Clang generates perfect codegen.

    andl $64, %edx
    xorl %eax, %eax
    cmovneq %rdi, %rsi
    cmovneq %rdi, %rax
    movq %rsi, %rdx

But with -DWORSE, Clang generates a dead-store "shr" instruction:

    andl $64, %edx
    xorl %eax, %eax
    shrl $6, %edx      // USELESS INSTRUCTION!
    cmovneq %rdi, %rsi
    cmovneq %rdi, %rax
    movq %rsi, %rdx

This is surprising, given that the only difference is using (0 == x) versus
(!x). I would expect these to have equivalent codegen.

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