| Summary: | GCC inserts redundant "test" instruction due to incorrect clobber | ||
|---|---|---|---|
| Product: | gcc | Reporter: | Jason Garrett-Glaser <darkshikari> |
| Component: | target | Assignee: | Not yet assigned to anyone <unassigned> |
| Status: | RESOLVED FIXED | ||
| Severity: | enhancement | CC: | astrange+gcc, gcc-bugs, jeffreyalaw |
| Priority: | P3 | Keywords: | missed-optimization |
| Version: | 4.6.0 | ||
| Target Milestone: | --- | ||
| Host: | Target: | i?86-*-* x86_64-*-* | |
| Build: | Known to work: | ||
| Known to fail: | Last reconfirmed: | 2011-12-30 00:00:00 | |
The problem is combine.
This:
int test2( int *b )
{
int b_ = *b;
b_--;
if( b_ == 0 ) {
*b = b_;
return foo();
}
*b = b_;
return 0;
}
works:
_test2:
LFB1:
movl (%rdi), %eax
decl %eax
je L7 <- uses decl
movl %eax, (%rdi)
xorl %eax, %eax
ret
.align 4,0x90
L7:
movl $0, (%rdi)
xorl %eax, %eax
jmp _foo
The original turns (*b)-- into load/dec/store/cmp - combine tries to combine dec/store which fails, but doesn't try dec/cmp.
Still happens with the new combine work (not that I really expected it to change). Confirmed.
Combine does:
Trying 7 -> 8:
Trying 9 -> 10:
But never 7 -> 9 which is needed for this to work.
This is for:
int foo();
int t;
int test( int b )
{
(b)--;
t = b;
if( b == 0 )
return foo();
return 0;
}
--- CUT --
Where 7, 8, 9, and 10 are:
(insn 7 3 8 2 (parallel [
(set (reg/v:SI 60 [ b ])
(plus:SI (reg/v:SI 62 [ b ])
(const_int -1 [0xffffffffffffffff])))
(clobber (reg:CC 17 flags))
]) t.c:6 253 {*addsi_1}
(expr_list:REG_DEAD (reg/v:SI 62 [ b ])
(expr_list:REG_UNUSED (reg:CC 17 flags)
(nil))))
(insn 8 7 9 2 (set (mem/c/i:SI (symbol_ref:DI ("t") <var_decl 0x7fedc8e8c140 t>) [2 t+0 S4 A32])
(reg/v:SI 60 [ b ])) t.c:7 64 {*movsi_internal}
(nil))
(insn 9 8 10 2 (set (reg:CCZ 17 flags)
(compare:CCZ (reg/v:SI 60 [ b ])
(const_int 0 [0]))) t.c:8 2 {*cmpsi_ccno_1}
(expr_list:REG_DEAD (reg/v:SI 60 [ b ])
(nil)))
(jump_insn 10 9 11 2 (set (pc)
(if_then_else (ne (reg:CCZ 17 flags)
(const_int 0 [0]))
(label_ref 16)
(pc))) t.c:8 599 {*jcc_1}
(expr_list:REG_DEAD (reg:CCZ 17 flags)
(expr_list:REG_BR_PROB (const_int 6102 [0x17d6])
(nil)))
-> 16)
Compare-elimination does the right thing and eliminates the redundant test on the trunk. Given the age of the BZ I didn't track it down any further than that. |
Take the following test case: int foo(); int test( int *b ) { (*b)--; if( *b == 0 ) return foo(); return 0; } On x86_64, with "-fomit-frame-pointer -O3 -c", gcc 4.6 compiles this to: 0: 8b 07 mov eax, [rdi] 2: 83 e8 01 sub eax, 0x1 5: 85 c0 test eax, eax 7: 89 07 mov [rdi], eax 9: 74 05 je 10 <test+0x10> b: 31 c0 xor eax, eax d: c3 ret 10: e9 00 00 00 00 jmp 15 <test+0x15> //foo As can be seen, gcc inserts a redundant "test" instruction. This problem is present in all versions of gcc I tested (3.4, 4.3.4, 4.4.1, and 4.6 SVN r160330). According to Alexander Strange on IRC: <astrange> the problem is that subl $1, %eax does (clobber (reg:CC 17 flags)) instead of (set (reg:CCZ 17 flags) (compare:CCZ .... <astrange> which i don't think is fixable in a real way, but i didn't write it