[LLVMbugs] [Bug 14366] New: Poor code generation for a simple inline assembly

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=14366

             Bug #: 14366
           Summary: Poor code generation for a simple inline assembly
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: FreeBSD
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: jkim at FreeBSD.org
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


Consider this:

% cat test.c
static __inline unsigned long
bsfq(unsigned long x)
{
        unsigned long r;

        __asm __volatile("bsfq %1,%0" : "=r" (r) : "rm" (x));
        return (r);
}

int
ffsl(long x)
{
        return (x == 0 ? x : (int)bsfq((unsigned long)x) + 1);
}

Clang generates really poor code:

% clang -O2 -c test.c
% objdump -d test.o

test.o:     file format elf64-x86-64-freebsd

Disassembly of section .text:

0000000000000000 <ffsl>:
   0:   48 85 ff                test   %rdi,%rdi
   3:   74 21                   je     26 <ffsl+0x26>
   5:   48 89 7c 24 f8          mov    %rdi,-0x8(%rsp)
   a:   48 0f bc 4c 24 f8       bsf    -0x8(%rsp),%rcx
  10:   48 c1 e1 20             shl    $0x20,%rcx
  14:   48 b8 00 00 00 00 01    mov    $0x100000000,%rax
  1b:   00 00 00 
  1e:   48 01 c8                add    %rcx,%rax
  21:   48 c1 e8 20             shr    $0x20,%rax
  25:   c3                      retq   
  26:   31 c0                   xor    %eax,%eax
  28:   c3                      retq   

Of course, GCC 4.2 does much better job. ;-)

% gcc -O2 -c test.c
% objdump -d test.o

test.o:     file format elf64-x86-64-freebsd

Disassembly of section .text:

0000000000000000 <ffsl>:
   0:   31 c0                   xor    %eax,%eax
   2:   48 85 ff                test   %rdi,%rdi
   5:   74 07                   je     e <ffsl+0xe>
   7:   48 0f bc c7             bsf    %rdi,%rax
   b:   83 c0 01                add    $0x1,%eax
   e:   f3 c3                   repz retq 

FYI, this problem was discussed here (warning: little lengthy):

http://docs.freebsd.org/cgi/mid.cgi?20121116181327.B1135

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