https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115863
--- Comment #11 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Uroš Bizjak from comment #10)
> Created attachment 58650 [details]
> Testcase that illustrates performance issue
Without ustrunc{m}{m}2 optab the loop in the testcase compiles to (gcc -O2):
.L7:
movl 12(%rsp), %eax
.L4:
testl %eax, %eax
jne .L2
movl $4294967295, %eax
cmpq %rax, %rbx
cmovbe %rbx, %rax
movl %eax, 12(%rsp)
subq %rax, %rbx
.L2:
leaq 12(%rsp), %rdi
call deflate
testl %eax, %eax
je .L7
where the relevant part of the _.optimized tree dump reads:
<bb 4> [local count: 536870913]:
_13 = MIN_EXPR <left_4, 4294967295>;
iftmp.0_6 = (unsigned int) _13;
stream.avail_out = iftmp.0_6;
left_15 = left_4 - _13;
and when ustrunc{m}{n} is present, the same loop compiles to:
.L7:
movl 12(%rsp), %eax
.L4:
testl %eax, %eax
jne .L2
movl $4294967295, %eax
movl %ebp, %edx
cmpq %rax, %rbx
cmovbe %rbx, %rax
cmpq %rbx, %rbp <---
cmovnc %ebx, %edx <---
subq %rax, %rbx
movl %edx, 12(%rsp)
.L2:
leaq 12(%rsp), %rdi
call deflate
testl %eax, %eax
je .L7
where the relevant part of the _.optimized tree dump reads:
<bb 4> [local count: 536870912]:
_12 = MIN_EXPR <left_3, 4294967295>;
iftmp.0_5 = .SAT_TRUNC (left_3);
stream.avail_out = iftmp.0_5;
left_14 = left_3 - _12;
Please note two new instructions in the second asm dump. These are expanded
from .SAT_TRUNC and are not present in the first asm dump.
The problem here is that the presence of ustrunc{m}{n}2 optab in i386.md
prevents some optimization involving .MIN_EXPR that would result in better
code.
