[llvm-bugs] [Bug 33757] New: GCC produces much better code for simple double loops than Clang
via llvm-bugs
llvm-bugs at lists.llvm.org
Wed Jul 12 02:37:44 PDT 2017
https://bugs.llvm.org/show_bug.cgi?id=33757
Bug ID: 33757
Summary: GCC produces much better code for simple double loops
than Clang
Product: libraries
Version: trunk
Hardware: All
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Loop Optimizer
Assignee: unassignedbugs at nondot.org
Reporter: gonzalobg88 at gmail.com
CC: llvm-bugs at lists.llvm.org
See here the code and asm comparison, code and assembly follow below:
https://gcc.godbolt.org/#g:!((g:!((g:!((h:codeEditor,i:(fontScale:1.1,j:1,source:'%23include+%3Cstdint.h%3E%0A%0Aint64_t+c_iter(int64_t+high)%0A%7B+++++++%0A++int64_t+total+%3D+0%3B%0A++int64_t+i%3B%0A++for+(i+%3D+1%3B+i+%3C%3D+high%3B+i%2B%2B)+%7B%0A++++while+(i+%25+2+!!%3D+0)+%7B%0A++++++i%2B%2B%3B%0A++++%7D%0A++++total+%2B%3D+i+*+2%3B%0A++%7D%0A++return+total%3B%0A%7D%0A%0Aint64_t+c_loop(int64_t+high)+%7B%0A++int64_t+total+%3D+0%3B%0A++int64_t+i%3B%0A++for+(i+%3D+1%3B+i+%3C%3D+high%3B+%2B%2Bi)+%7B%0A++++if+(i+%25+2+%3D%3D+0)+%7B%0A++++++total+%2B%3D+i+*+2%3B%0A++++%7D%0A++%7D%0A++return+total%3B%0A%7D'),l:'5',n:'0',o:'C%2B%2B+source+%231',t:'0')),k:33.03381678958197,l:'4',n:'0',o:'',s:0,t:'0'),(g:!((h:compiler,i:(compiler:clang400,filters:(b:'0',commentOnly:'0',directives:'0',intel:'0'),options:'-O2+-fno-unroll-loops',source:1),l:'5',n:'0',o:'x86-64+clang+4.0.0+(Editor+%231,+Compiler+%231)',t:'0')),k:33.632849877084716,l:'4',n:'0',o:'',s:0,t:'0'),(g:!((h:compiler,i:(compiler:g71,filters:(b:'0',commentOnly:'0',directives:'0',intel:'0'),options:'-O2',source:1),l:'5',n:'0',o:'x86-64+gcc+7.1+(Editor+%231,+Compiler+%232)',t:'0')),k:33.33333333333333,l:'4',n:'0',o:'',s:0,t:'0')),l:'2',n:'0',o:'',t:'0')),version:4
Code:
#include <stdint.h>
int64_t c_iter(int64_t high)
{
int64_t total = 0;
int64_t i;
for (i = 1; i <= high; i++) {
while (i % 2 != 0) {
i++;
}
total += i * 2;
}
return total;
}
int64_t c_loop(int64_t high) {
int64_t total = 0;
int64_t i;
for (i = 1; i <= high; ++i) {
if (i % 2 == 0) {
total += i * 2;
}
}
return total;
}
GCC assembly:
c_iter(long):
test rdi, rdi
jle .L4
sub rdi, 1
mov edx, 4
xor eax, eax
shr rdi
lea rcx, [8+rdi*4]
.L3:
add rax, rdx
add rdx, 4
cmp rcx, rdx
jne .L3
rep ret
.L4:
xor eax, eax
ret
c_loop(long):
test rdi, rdi
jle .L12
add rdi, 1
mov edx, 1
xor eax, eax
jmp .L9
.L11:
lea rcx, [rax+rdx*2]
test dl, 1
cmove rax, rcx
.L9:
add rdx, 1
cmp rdx, rdi
jne .L11
rep ret
.L12:
xor eax, eax
ret
Clang assembly -O2 -funroll-loops:
c_iter(long): # @c_iter(long)
xor eax, eax
test rdi, rdi
jle .LBB0_5
mov ecx, 1
.LBB0_2: # =>This Loop Header: Depth=1
lea rax, [rax + 2*rcx - 2]
.LBB0_3: # Parent Loop BB0_2 Depth=1
mov edx, ecx
inc rcx
add rax, 2
test dl, 1
jne .LBB0_3
lea rdx, [rcx - 1]
cmp rdx, rdi
jl .LBB0_2
.LBB0_5:
ret
c_loop(long): # @c_loop(long)
test rdi, rdi
jle .LBB1_1
lea r8, [rdi - 1]
mov r9, rdi
and r9, 3
je .LBB1_3
xor edx, edx
mov ecx, 2
xor eax, eax
.LBB1_5: # =>This Inner Loop Header: Depth=1
inc rdx
test dl, 1
mov esi, 0
cmove rsi, rcx
add rax, rsi
add rcx, 2
cmp r9, rdx
jne .LBB1_5
inc rdx
cmp r8, 3
jae .LBB1_8
jmp .LBB1_10
.LBB1_1:
xor eax, eax
ret
.LBB1_3:
xor eax, eax
mov edx, 1
cmp r8, 3
jb .LBB1_10
.LBB1_8:
neg rdi
lea rcx, [rdx + rdx + 6]
lea rdx, [rdx + 3]
xor r8d, r8d
.LBB1_9: # =>This Inner Loop Header: Depth=1
lea r9d, [rdx - 3]
lea rsi, [rcx - 6]
test r9b, 1
cmovne rsi, r8
add rsi, rax
lea r9d, [rdx - 2]
lea rax, [rcx - 4]
test r9b, 1
cmovne rax, r8
add rax, rsi
lea r9d, [rdx - 1]
lea rsi, [rcx - 2]
test r9b, 1
cmovne rsi, r8
add rsi, rax
test dl, 1
mov eax, 0
cmove rax, rcx
add rax, rsi
lea rsi, [rdi + rdx + 4]
add rdx, 4
add rcx, 8
cmp rsi, 4
jne .LBB1_9
.LBB1_10:
ret
Clang assembly: -O2 -fno-unroll-loops
c_iter(long): # @c_iter(long)
xor eax, eax
test rdi, rdi
jle .LBB0_5
mov ecx, 1
.LBB0_2: # =>This Loop Header: Depth=1
lea rax, [rax + 2*rcx - 2]
.LBB0_3: # Parent Loop BB0_2 Depth=1
mov edx, ecx
inc rcx
add rax, 2
test dl, 1
jne .LBB0_3
lea rdx, [rcx - 1]
cmp rdx, rdi
jl .LBB0_2
.LBB0_5:
ret
c_loop(long): # @c_loop(long)
test rdi, rdi
jle .LBB1_1
xor ecx, ecx
mov edx, 2
xor eax, eax
.LBB1_3: # =>This Inner Loop Header: Depth=1
inc rcx
test cl, 1
mov esi, 0
cmove rsi, rdx
add rax, rsi
add rdx, 2
cmp rdi, rcx
jne .LBB1_3
ret
.LBB1_1:
xor eax, eax
ret
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