[Bug rtl-optimization/48877] Inline asm for rdtsc generates silly code

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48877

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.05.05 06:50:18
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |vmakarov at gcc dot gnu.org
     Ever Confirmed|0                           |1

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-05-05 06:50:18 UTC ---
If you use return __builtin_ia32_rdtsc (); instead, both 4.6 and 4.7 generate:
        rdtsc
        salq    $32, %rdx
        orq     %rdx, %rax
        ret
Current GCC trunk generates:
#APP
# 7 "pr48877.c" 1
        rdtsc
# 0 "" 2
#NO_APP
        salq    $32, %rdx
        orq     %rax, %rdx
        movq    %rdx, %rax
        ret
for the asm testcase, which isn't as bad as 4.6, but isn't perfect.  What
matters for IRA is which pseudo is LHS/RHS1 and which is RHS2 on the orq insn,
for the builtin version LHS/RHS1 is the pseudo set by the unspecv with "=a"
constraint, for the asm version it is the LHS from the shift insn.