[LLVMbugs] [Bug 20197] New: Inline asm constraint alternatives ignored

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Thu Jul 3 07:40:05 PDT 2014


            Bug ID: 20197
           Summary: Inline asm constraint alternatives ignored
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: benny.kra at gmail.com
                CC: echristo at gmail.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

When multiple alternatives in an inline asm constraint are given we ignore all
of them but the most "general". This gives nasty artifacts in the code.

int bsr(unsigned v) {
  int ret;
  __asm__("bsr %1, %0" : "=&r"(ret) : "rm"(v) : "cc");
  return ret;

$ clang -O3 -S -o - t.c
    movl    %edi, -4(%rsp)
    bsrl    -4(%rsp), %eax

The spilling is totally unnecessary. GCC gets this one right. On 32 bit x86
it's even worse:

$ clang -O3 -S -o - t.c -m32
    pushl    %eax
    movl    8(%esp), %eax
    movl    %eax, (%esp)
    bsrl    (%esp), %eax
    popl    %edx

GCC knows a better way:
$ gcc-4.8 -O3 -S -o - t.c -m32
    bsr 4(%esp), %eax

The constraint "g" is just as bad, being translated into "imr" internally.

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