https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112723
Bug ID: 112723
Summary: Missed optimization for invariants 'c+c' when c +=
-2147483648 and c is a global variable
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 noticed that maybe there is a missed optimization for invariants
'c+c' when c+= -2147483648 and c is a global variable.
Such missed optimization affects many related optimizations, such as LICM
(where we found this issue), CSE, and others.
To describe this issue more clearly, we show an example of CSE. And we attach a
link to an example of LICM at the end.
In the following, the value of c+c is invariant between the two expressions.
In this example, c is the global variable.
https://godbolt.org/z/Y3djqKWjh
int a, b, c;
void test() {
a = c + c;
c += -2147483648;
b = c + c;
}
But GCC -O3 -fwrapv or GCC -O3:
test():
movl c(%rip), %eax
leal (%rax,%rax), %edx
addl $-2147483648, %eax
movl %eax, c(%rip)
addl %eax, %eax #Redundant computation
movl %edx, a(%rip)
movl %eax, b(%rip)
ret
Expected code (Clang):
test(): # @test()
mov eax, dword ptr [rip + c]
lea ecx, [rax + rax]
mov dword ptr [rip + a], ecx
add eax, -2147483648
mov dword ptr [rip + c], eax
mov dword ptr [rip + b], ecx
ret
When c appears as a function argument, it works as expected:
https://godbolt.org/z/Tf6hP6M9W
int a, b;
void test(int c) {
a = c + c;
c += -2147483648;
b = c + c;
}
GCC -O3:
test(int):
leal (%rdi,%rdi), %eax
movl %eax, a(%rip)
movl %eax, b(%rip)
ret
An example of LICM affected by this: https://godbolt.org/z/5T89TWaG9
Thank you very much for your time and effort! We look forward to hearing from
you.
