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References: <bug-39942-17483@http.gcc.gnu.org/bugzilla/>
Subject: [Bug target/39942] Nonoptimal code - leaveq; xchg   %ax,%ax; retq
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------- Comment #6 from vvv at ru dot ru  2009-04-28 21:18 -------
Let's compile file test.c
//#file test.c

extern int F(int m);

void func(int x)
{
        int u = F(x);
        while (u)
                u = F(u)*3+1;
}


# gcc -o t.out test.c -c -O2
# objdump -d t.out

t.out:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <func>:
   0:   48 83 ec 08             sub    $0x8,%rsp
   4:   e8 00 00 00 00          callq  9 <func+0x9>
   9:   85 c0                   test   %eax,%eax
   b:   89 c7                   mov    %eax,%edi
   d:   74 0e                   je     1d <func+0x1d>
   f:   90                      nop    
  10:   e8 00 00 00 00          callq  15 <func+0x15>
  15:   8d 7c 40 01             lea    0x1(%rax,%rax,2),%edi
  19:   85 ff                   test   %edi,%edi
  1b:   75 f3                   jne    10 <func+0x10>
  1d:   48 83 c4 08             add    $0x8,%rsp
  21:   0f 1f 80 00 00 00 00    nopl   0x0(%rax)   <---- nonoptimal
  28:   c3                      retq   


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39942