| Summary: | [4.7/4.8/4.9 Regression] GCC produces unnecessary code on -O2 and -O3 levels | ||
|---|---|---|---|
| Product: | gcc | Reporter: | Dmitry Gorbachev <d.g.gorbachev> |
| Component: | rtl-optimization | Assignee: | Not yet assigned to anyone <unassigned> |
| Status: | RESOLVED FIXED | ||
| Severity: | normal | CC: | jakub |
| Priority: | P4 | Keywords: | missed-optimization |
| Version: | 4.6.0 | ||
| Target Milestone: | 4.7.4 | ||
| Host: | i686-pc-linux-gnu | Target: | i686-pc-linux-gnu |
| Build: | i686-pc-linux-gnu | Known to work: | |
| Known to fail: | Last reconfirmed: | 2013-03-05 00:00:00 | |
Why do you think it is wrong? If %esi is non-zero upon entry, then it is just moved to %eax and back, so it isn't changed, if it was zero upon entry, then 0 is loaded into it.
The reason for the extra code is that pre attempts to optimize it (register vars are not SSA vars), so we get at *.optimized:
int prephitmp.4;
int r.0;
<bb 2>:
r.0_1 = r;
if (r.0_1 != 0)
goto <bb 3>;
else
goto <bb 4>;
<bb 3>:
__asm__("sar %0" : "=r" r : "0" r.0_1);
prephitmp.4_2 = r;
<bb 4>:
# prephitmp.4_8 = PHI <0(2), prephitmp.4_2(3)>
__asm__("sar %0" : "=r" r : "0" prephitmp.4_8);
and RTL optimizations aren't able to undo that. I doubt anything can be done about this easily, with -fno-tree-pre you get your expected output.
Ah, yes; changed the summary. 4.4 branch is being closed, moving to 4.5.4 target. (In reply to comment #1) > and RTL optimizations aren't able to undo that. I doubt anything can > be done about this easily. Perhaps tree-ssa-uncprop should handle this. FWIW, I've always thought that constant propagation into PHI arguments was a mistake... GCC 4.6.4 has been released and the branch has been closed. I can't reproduce the bug with GCC 4.7.3 and 4.8.1. Apparently fixed (probably by IRA). |
$ cat > testcase.c register int r asm("esi"); void foo(void) { if (r) __asm__("sar\t%0" : "+r" (r)); __asm__("sar\t%0" : "+r" (r)); } ^D $ gcc -O2 -S testcase.c $ cat testcase.s [...] foo: .LFB0: xorl %eax, %eax testl %esi, %esi je .L2 #APP # 6 "testcase.c" 1 sar %esi # 0 "" 2 #NO_APP movl %esi, %eax .L2: movl %eax, %esi #APP # 8 "testcase.c" 1 sar %esi # 0 "" 2 #NO_APP ret [...]