https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111560
Bug ID: 111560
Summary: Missed optimization of available expression
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 Available Expression) that GCC
may have missed. We would greatly appreicate if you can take a look and let us
know what you think.
Here are three examples.
Example 1:
https://godbolt.org/z/eYcWocsPf
Given the following code:
```c++
int test()
{
int a,b,c,d,e,f,g;
cin>>a>>b>>c>>d>>g;
e=a+b+c; //line 5
f=d+b+c; //"b+c" can be replaced with the value at line 5
cout<<e<<f;
return 0;
}
```
We note that `b+c` at line 6 in the test code can be replaced with the value at
line 5, but gcc-trunk -O3 does not:
```asm
test():
push rbx
mov edi, OFFSET FLAT:_ZSt3cin
sub rsp, 32
lea rsi, [rsp+12]
call std::basic_istream<char, std::char_traits<char>
>::operator>>(int&)
lea rsi, [rsp+16]
mov rdi, rax
call std::basic_istream<char, std::char_traits<char>
>::operator>>(int&)
lea rsi, [rsp+20]
mov rdi, rax
call std::basic_istream<char, std::char_traits<char>
>::operator>>(int&)
lea rsi, [rsp+24]
mov rdi, rax
call std::basic_istream<char, std::char_traits<char>
>::operator>>(int&)
lea rsi, [rsp+28]
mov rdi, rax
call std::basic_istream<char, std::char_traits<char>
>::operator>>(int&)
mov esi, DWORD PTR [rsp+16]
mov ebx, DWORD PTR [rsp+24]
mov edi, OFFSET FLAT:_ZSt4cout
mov eax, DWORD PTR [rsp+20]
add ebx, esi
add esi, DWORD PTR [rsp+12]
add esi, eax
add ebx, eax
call std::basic_ostream<char, std::char_traits<char>
>::operator<<(int)
mov esi, ebx
mov rdi, rax
call std::basic_ostream<char, std::char_traits<char>
>::operator<<(int)
add rsp, 32
xor eax, eax
pop rbx
ret
```
Example 2:
https://godbolt.org/z/o61sx6dGh
Given the following code:
```c++
int var_13; int var_14;
void test(int var_0, int var_4, int var_5) {
var_13 = var_0 + var_4 + var_5; //line 3
var_14 = var_4 + var_5; //"var_4 + var_5" can be replaced with the value
at line 3
}
```
We note that `var_4 + var_5` at line 4 in the test code can be replaced with
the value at line 3, but gcc-trunk -O3 does not:
```asm
test(int, int, int):
add edi, esi
add esi, edx
add edi, edx
mov DWORD PTR var_14[rip], esi
mov DWORD PTR var_13[rip], edi
ret
var_14:
.zero 4
var_13:
.zero 4
```
Example 3:
https://godbolt.org/z/caYT9E6Mz
Similar to example 2, but this is an example that might involve shifting
operations:
```c++
int var_14,var_27;
void test(int var_1, int var_3) {
var_14 = var_1 + var_1;
var_27 = var_3 + var_1 + var_1;
}
```
```asm
test(int, int):
lea eax, [rdi+rdi]
mov DWORD PTR var_14[rip], eax
lea eax, [rsi+rdi*2]
mov DWORD PTR var_27[rip], eax
ret
var_27:
.zero 4
var_14:
.zero 4
```
Thank you very much for your time and effort! We look forward to hearing from
you.
