[Bug tree-optimization/114317] New: Missing optimization for multiple condition statements

[Bug tree-optimization/114317] New: Missing optimization for multiple condition statements

[carnet at student dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114317

            Bug ID: 114317
           Summary: Missing optimization for multiple condition statements
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: carnet at student dot ethz.ch
  Target Milestone: ---

GCC -O3 is not able to optimize the [b && (g = 0)] statement even though g is
static and 0. So the statement does not change the value of g. This further
prevents the optimiziation of the later statements.

https://godbolt.org/z/r7P7qajYn

Source:
int b;
int *e = &b;
static int g = 0;

int main() {
    b && (g = 0);
    if (3 - g){
        *e = b;
    }
    if (253 - (9 | g)){
        *e = b;
    }
    return *e;
}

x86 -O3 Assembly:
main:
        movl    b(%rip), %eax
        testl   %eax, %eax
        je      .L2
        movq    e(%rip), %rdx
        xorl    %edi, %edi
        movl    %edi, g(%rip)
        movl    %eax, (%rdx)
        movl    b(%rip), %eax
.L3:
        movl    %eax, (%rdx)
        ret
.L2:
        movl    g(%rip), %ecx
        movq    e(%rip), %rdx
        cmpl    $3, %ecx
        je      .L3
        xorl    %esi, %esi
        orl     $9, %ecx
        movl    %esi, (%rdx)
        cmpl    $253, %ecx
        jne     .L8
        ret
.L8:
        movl    b(%rip), %eax
        jmp     .L3
e:
        .quad   b
b:
        .zero   4

[Bug ipa/114317] Missing optimization for multiple condition statements

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114317

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-03-12
          Component|tree-optimization           |ipa
             Status|UNCONFIRMED                 |NEW
                 CC|                            |hubicka at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  I think this is known and there's possibly a duplicate.  IPA
reference doesn't consider writing the same value as in the initializer
as a way to turn the variable read-only (it would need to elide the stores
of course).

[Bug ipa/114317] Missing optimization for multiple condition statements

[hubicka at ucw dot cz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114317

--- Comment #2 from Jan Hubicka <hubicka at ucw dot cz> ---
> (it would need to elide the stores of course).

We do have way to elide stores, since we can optimize out write-only
values.  What we do not have readilly available is the value written to
a reference (ipa-refs only connects variables/functions to statements
that write/load/take address).  

I think the plan is to add summary template for references, like we have
for callgraph edges and symbols.  Then we can keep track of values and
do optimizations like this.  It is on my todo for quite some time ;)