[llvm-bugs] [Bug 32752] New: ThinLTO missing optimization on Globals

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=32752

            Bug ID: 32752
           Summary: ThinLTO missing optimization on Globals
           Product: new-bugs
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: mehdi.amini at apple.com
                CC: llvm-bugs at lists.llvm.org

This examples shows some current limitations of ThinLTO with globals:

// globalopt0.c
int g0 = 0, g1 = 22, g2;
extern void foo1(void);
int main() {
  g0 = 1;
  g1 = 22;
  g2 = 1;
  foo1();
  return g0 + g1;
}

// globalopt1.c
extern int g0, g1, g2;
void __attribute__((noinline)) foo1(void) {
  g0 = 0;
  g1 = 22;
  g2 = 4;
}


Build with:

  $ clang globalopt0.c -c -flto=thin -O3
  $ clang globalopt1.c -c -flto=thin -O3
  $ clang globalopt0.o globalopt1.o -O3 -o globalopt -Wl,-save-temps


Some potential optimizations:

- g0’s type can be changed to i1 because only two values (0 or 1) are stored to
it.
- g1 should be optimized away because only one value (22) is stored.
- g2 should be optimized away because it’s store-only.

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