http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50213
Bug #: 50213
Summary: Regression in space-optimized code relative to 4.5.x
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: big...@acm.org
Some change between 4.5.x and 4.6.x decreased generated code quality,
especially noticeable with -Os, for the following function:
int checkTLV (unsigned char* tlv)
{
unsigned int csum;
extern unsigned int __info_segment_size;
const unsigned int* p = (const unsigned int*)(tlv + __info_segment_size);
csum = 0;
while (--p > (const unsigned int*)tlv)
csum ^= *p;
csum += *p;
return 0 == csum;
}
In 4.5.2 -Os, the loop has an 11 byte body:
11 000c EB02 jmp .L2 # 52 jump [length = 2]
12 .L3:
13 000e 3310 xorl (%rax), %edx # 15 *xorsi_1/1
[length = 2]
14 .L2:
15 0010 4883E804 subq $4, %rax # 18 *adddi_1/1
[length = 4]
16 0014 4839F8 cmpq %rdi, %rax # 20 *cmpdi_1/1
[length = 3]
17 0017 77F5 ja .L3 # 21 *jcc_1 [length = 2]
while in 4.6.1 -Os the loop has a 15 byte body:
11 000b EB06 jmp .L2 # 66 jump [length = 2]
12 .L3:
13 000d 3350FC xorl -4(%rax), %edx # 29 *xorsi_1/1
[length = 3]
14 0010 4889C8 movq %rcx, %rax # 21
*movdi_internal_rex64/2 [length = 3]
15 .L2:
16 0013 488D48FC leaq -4(%rax), %rcx # 57 *lea_1 [length
= 4]
17 0017 4839F9 cmpq %rdi, %rcx # 34 *cmpdi_1/1
[length = 3]
18 001a 77F1 ja .L3 # 35 *jcc_1 [length = 2]
My hypothesis is that the label L2 is preventing the pseudo registers for the
loop variable and the memory access from being merged into the same hardware
register. This is not specific to the x86 back-end: I noticed it in an
unsupported back-end, where the suboptimal register allocation causes a spill.
The current trunk (svn 178132) generates the same suboptimal code as 4.6.1.
