[LLVMbugs] [Bug 22357] New: missed opportunities in reassociation or instcombine

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=22357

            Bug ID: 22357
           Summary: missed opportunities in reassociation or instcombine
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: wujingyue at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 13742
  --> http://llvm.org/bugs/attachment.cgi?id=13742&action=edit
IR for the example

void foo(int a, int b, int c, int *x, int *y) {
  *x = (a + b);
  *y = (a + 2) + b;
}

I would love to see the above example be optimized to:

void foo(int a, int b, int c, int *x, int *y) {
  int t = a + b;
  *x = t;
  int t2 = t + 2;
  *y = t2;
}

However, this is not happening with the current LLVM.

I wonder how we can add this enhancement. It seems related to reassociation,
instcombine, and maybe even GVN? 

Jingyue

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