[llvm-bugs] [Bug 52481] New: Instconsistent sdiv and srem by pow2 codegen under Oz

via llvm-bugs llvm-bugs at lists.llvm.org
Thu Nov 11 12:54:40 PST 2021


            Bug ID: 52481
           Summary: Instconsistent sdiv and srem by pow2 codegen under Oz
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: craig.topper at gmail.com
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, pengfei.wang at intel.com,
                    spatel+llvm at rotateright.com

There seems to be an ordering issue in our handling of sdiv/srem by constant
handling under -Oz.

These two test cases produce different code just from reordering the div and

int foo(int x, int *rem) {
  *rem = x % 256;
  return x / 256;

int bar(int x, int *div) {
  *div = x / 256;
  return x % 256;

foo:                                    # @foo
        mov     ecx, 256
        mov     eax, edi
        idiv    ecx
        mov     ecx, eax
        shl     ecx, 8
        sub     edi, ecx
        mov     dword ptr [rsi], edi
bar:                                    # @bar
        mov     eax, edi
        mov     ecx, 256
        idiv    ecx
        mov     dword ptr [rsi], eax
        mov     eax, edx

bar is using the remainder from the idiv, while foo is using the quotient to
compute the remainder.

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