http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46076
Summary: [4.6 regression] constant propogation and compile-time
math no longer happening versus 4.4 and 4.5
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: [email protected]
ReportedBy: [email protected]
For the following code:
#include <stdio.h>
typedef unsigned char(*Calculable)(void);
static unsigned char one() { return 1; }
static unsigned char two() { return 2; }
static void print(Calculable calculate)
{
printf("%d\n", calculate());
printf("+1: %d\n", calculate() + 1);
}
int main()
{
print(one);
print(two);
return 0;
}
GCC 4.4 (Ubuntu/Linaro 4.4.4-14ubuntu5) and 4.5(Ubuntu/Linaro 4.5.1-7ubuntu2),
when using -O3 on amd64 using Ubuntu 10.10's packages, produce the following
code:
00000000004005a0 <main>:
4005a0: 48 83 ec 08 sub rsp,0x8
4005a4: ba 01 00 00 00 mov edx,0x1
4005a9: be 00 07 40 00 mov esi,0x400700
4005ae: bf 01 00 00 00 mov edi,0x1
4005b3: 31 c0 xor eax,eax
4005b5: e8 ae fe ff ff call 400468 <__printf_...@plt>
4005ba: ba 02 00 00 00 mov edx,0x2
4005bf: be fc 06 40 00 mov esi,0x4006fc
4005c4: bf 01 00 00 00 mov edi,0x1
4005c9: 31 c0 xor eax,eax
4005cb: e8 98 fe ff ff call 400468 <__printf_...@plt>
4005d0: ba 02 00 00 00 mov edx,0x2
4005d5: be 00 07 40 00 mov esi,0x400700
4005da: bf 01 00 00 00 mov edi,0x1
4005df: 31 c0 xor eax,eax
4005e1: e8 82 fe ff ff call 400468 <__printf_...@plt>
4005e6: ba 03 00 00 00 mov edx,0x3
4005eb: be fc 06 40 00 mov esi,0x4006fc
4005f0: bf 01 00 00 00 mov edi,0x1
4005f5: 31 c0 xor eax,eax
4005f7: e8 6c fe ff ff call 400468 <__printf_...@plt>
4005fc: 31 c0 xor eax,eax
4005fe: 48 83 c4 08 add rsp,0x8
400602: c3 ret
where the constants are correctly propogated and the compile-time math occurs.
With Ubuntu 10.10's gcc-snapshot of 4.6 (Ubuntu 20101004-0ubuntu1) or an
alternate 4.6 version (Sourcery G++ 2010.09-31), neither of these things
happens, even with -fwhole-program:
0000000000400510 <_ZL3onev>:
400510: b8 01 00 00 00 mov eax,0x1
400515: c3 ret
400516: eb 08 jmp 400520 <_ZL3twov>
400518: 90 nop
400519: 90 nop
40051a: 90 nop
40051b: 90 nop
40051c: 90 nop
40051d: 90 nop
40051e: 90 nop
40051f: 90 nop
0000000000400520 <_ZL3twov>:
400520: b8 02 00 00 00 mov eax,0x2
400525: c3 ret
400526: eb 08 jmp 400530 <_ZL5printPFhvE>
400528: 90 nop
400529: 90 nop
40052a: 90 nop
40052b: 90 nop
40052c: 90 nop
40052d: 90 nop
40052e: 90 nop
40052f: 90 nop
0000000000400530 <_ZL5printPFhvE>:
400530: 53 push rbx
400531: 48 89 fb mov rbx,rdi
400534: ff d7 call rdi
400536: bf 74 06 40 00 mov edi,0x400674
40053b: 0f b6 f0 movzx esi,al
40053e: 31 c0 xor eax,eax
400540: e8 b3 fe ff ff call 4003f8 <pri...@plt>
400545: ff d3 call rbx
400547: 5b pop rbx
400548: 0f b6 f0 movzx esi,al
40054b: bf 70 06 40 00 mov edi,0x400670
400550: 31 c0 xor eax,eax
400552: 83 c6 01 add esi,0x1
400555: e9 9e fe ff ff jmp 4003f8 <pri...@plt>
40055a: eb 04 jmp 400560 <main>
40055c: 90 nop
40055d: 90 nop
40055e: 90 nop
40055f: 90 nop
0000000000400560 <main>:
400560: 48 83 ec 08 sub rsp,0x8
400564: bf 10 05 40 00 mov edi,0x400510
400569: e8 c2 ff ff ff call 400530 <_ZL5printPFhvE>
40056e: bf 20 05 40 00 mov edi,0x400520
400573: e8 b8 ff ff ff call 400530 <_ZL5printPFhvE>
400578: 31 c0 xor eax,eax
40057a: 48 83 c4 08 add rsp,0x8
40057e: c3 ret
