[llvm-bugs] [Bug 39657] New: suboptimal address calculation for aarch64

via llvm-bugs llvm-bugs at lists.llvm.org
Tue Nov 13 19:33:10 PST 2018


            Bug ID: 39657
           Summary: suboptimal address calculation for aarch64
           Product: new-bugs
           Version: unspecified
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: froydnj at gmail.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

This hackish code was extracted from some real code that was attempting to be

int f(char* p, long x)
  int i;
  __builtin_memcpy(&i, (int*)(p + (-x - 1)*4), sizeof(i));
  return i;

clang -O2 trunk generates, according to gcc.godbolt.org:

        lsl     x8, x1, #2
        eor     x8, x8, #0xfffffffffffffffc
        ldr     w0, [x0, x8]

whereas the preferred code would be (GCC 6.3.0, according to gcc.godbolt.org):

        mvn     x1, x1
        ldr     w0, [x0, x1, lsl 2]

where the shift has been folded into the load instruction.

The same phenomenon is visible on x86-64.  clang:

        shlq    $2, %rsi
        xorq    $-4, %rsi
        movl    (%rdi,%rsi), %eax


        notq    %rsi
        movl    (%rdi,%rsi,4), %eax

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