[llvm-bugs] [Bug 47661] New: Reuse flags from neg to eliminate test instruction

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=47661

            Bug ID: 47661
           Summary: Reuse flags from neg to eliminate test instruction
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

foo(unsigned long long const*, unsigned long long*, unsigned long long):       
                   # @foo(unsigned long long const*, unsigned long long*,
unsigned long long)
        test    rdx, rdx
        je      .LBB0_3
        neg     rdx
.LBB0_2:                                # =>This Inner Loop Header: Depth=1
        mov     rax, qword ptr [rdi + 8*rdx]
        and     qword ptr [rsi + 8*rdx], rax
        inc     rdx
        jne     .LBB0_2
.LBB0_3:
        ret


Clang -Os:
foo(unsigned long long const*, unsigned long long*, unsigned long long):       
                   # @foo(unsigned long long const*, unsigned long long*,
unsigned long long)
        test    rdx, rdx
        je      .LBB0_3
        neg     rdx
.LBB0_2:                                # =>This Inner Loop Header: Depth=1
        mov     rax, qword ptr [rdi + 8*rdx]
        and     qword ptr [rsi + 8*rdx], rax
        inc     rdx
        jne     .LBB0_2
.LBB0_3:
        ret

GCC -O2:
foo(unsigned long long const*, unsigned long long*, unsigned long long):
        neg     rdx
        je      .L1
.L3:
        mov     rax, QWORD PTR [rdi+rdx*8]
        and     QWORD PTR [rsi+rdx*8], rax
        add     rdx, 1
        jne     .L3
.L1:
        ret


https://godbolt.org/z/av85T1

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