https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111564
Bug ID: 111564
Summary: Missed optimization of Loop Unswitch
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: 652023330028 at smail dot nju.edu.cn
Target Milestone: ---
Hello, we found some optimizations (regarding Loop Unswitch) that GCC may have
missed. We would greatly appreicate if you can take a look and let us know what
you think.
Here are two examples.
Example 1:
https://godbolt.org/z/G3xGvGxq1
Given the following code:
```c++
extern int var_21;
extern int var_25;
extern int var_28;
extern int var_29;
void test(int var_0, int var_1, int var_2, int var_3, int var_4, int var_5, int
var_6, int var_7, int var_8, int var_9, int var_10, int var_11, int var_12, int
var_13, int var_14, int var_15, int var_16, int var_17) {
for (int i_0 = 0/*0*/; i_0 < ((((((((var_17)) ? (((819565140) + (var_9))) :
((((var_16)) ? (2141366111) : (var_17)))))) ? (((((((var_4)) ? (var_16) :
(var_17)))) ? (779921519) : (((var_1) + (var_8))))) : (((((((var_10)) ?
(var_12) : (var_5)))) ? ((((var_15)) ? (var_16) : (var_12))) : ((((var_3)) ?
(var_0) : (var_11))))))) - (779921500))/*19*/; i_0 += 4/*4*/)
{
var_21 += var_17;
if (var_3)
{
var_25 += var_17 ? -212529188 : (((var_6) ? (var_5 ? 2029341098 :
var_13) : var_11));
var_28 += var_8 ? ((var_10 ? (var_13 - var_1) : var_14 ? var_17 :
var_8)) : (var_6 ? 942541005 : var_1 / var_9);
var_29 += var_2 ? var_6 : ((var_9 ? var_8 : (var_11 ? var_2 :
var_15)));
}
}
}
```
Because `var_3` is a loop invariant so we can hoist the if condition out of the
loop, but gcc-trunk -O3 dose not (main parts):
```asm
test(int, int, int, int, int):
...
.L154:
cmp edx, ecx
jle .L9
test r10d, r10d
je .L153
...
.L153:
add ecx, 4
mov eax, 1
jmp .L154
```
Example 2:
https://godbolt.org/z/WcMnz97jv
Given the following code:
```c++
extern int var_13;
extern int var_15;
extern int var_17;
extern int var_19;
extern int var_20;
extern int var_24;
void test(int var_0, int var_1, int var_2, int var_3, int var_4, int var_5, int
var_6, int var_7, int var_8, int var_9, int var_10) {
for (int i_0 = ((var_9) + (1516547146))/*0*/; i_0 < (((-((-((((var_4)) ?
(var_6) : (var_8))))))) + (960893662))/*16*/; i_0 += ((var_0) +
(1994872666))/*1*/)
{
if (var_1)
{
var_13 += (var_0 ? var_8 : var_7) ? (var_0 ? var_7 : var_4) :
((var_7 ? var_1 : var_2) ? 1 : 2);
}
var_15 += (var_0 ? var_9 : var_6) ? (var_4 ? !var_5 : (var_7 ? var_3 :
var_9)) : var_8;
var_17 += var_3 ? ((var_4 ? var_5 : var_8) ? var_3 : var_1) : (var_5 ?
var_8 : var_0);
var_19 += (var_1 ? var_8 : var_2) ? (var_1 ? (var_7 ? var_3 : var_9) :
!var_3) : var_9;
var_20 += var_10 ? var_3 : (var_4 ? var_10 : var_8);
var_24 += var_0 ? -var_5 : (var_8 ? (var_3 ? var_8 : var_1) : (var_6 ?
var_6 : var_5));
}
}
```
Similarly, `var_1` is an loop invariant and should be hoisted out of the loop
as well.
But gcc-trunk -O3 does not:
```asm
test(int, int, int, int, int):
...
.L273:
test ecx, ecx
je .L87
mov r8d, DWORD PTR [rsp+64]
add DWORD PTR [rsp-28], r8d
test ebp, ebp
jne .L120
add DWORD PTR [rsp-32], ecx
test edx, edx
jne .L271
.L121:
mov BYTE PTR [rsp-14], 1
.L89:
mov ebx, DWORD PTR [rsp-12]
test r12d, r12d
cmovne ebx, ecx
.L92:
add r9d, ebx
test r10d, r10d
je .L100
.L102:
cmp BYTE PTR [rsp-13], 0
mov ebx, edx
jne .L103
.L99:
mov ebx, ebp
.L103:
mov r8d, DWORD PTR [rsp+88]
add edi, ebx
test r8d, r8d
je .L104
.L274:
mov ebx, r11d
add eax, r13d
add esi, edx
sub ebx, r12d
cmp r15d, eax
jle .L272
.L130:
mov r11d, ebx
test r10d, r10d
jne .L273
...
```
Thank you very much for your time and effort! We look forward to hearing from
you.
