[llvm-bugs] [Bug 40125] New: Poor code generated for __builtin_sub_overflow with constant RHS

via llvm-bugs llvm-bugs at lists.llvm.org
Thu Dec 20 15:24:19 PST 2018


            Bug ID: 40125
           Summary: Poor code generated for __builtin_sub_overflow with
                    constant RHS
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: craig.topper at gmail.com
                CC: llvm-bugs at lists.llvm.org

This generates some impressively bad code considering its equivalent to
checking if x is 0.

void bar();

void foo(unsigned x) {
  unsigned y;
  if (__builtin_sub_overflow(x, 1, &y))

X86 assembly 

        mov     eax, edi
        add     rax, -1
        movabs  rcx, 8589934591
        and     rcx, rax
        add     edi, -1
        cmp     rcx, rdi
        jne     .LBB0_2
        jmp     bar()                 # TAILCALL


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