[LLVMbugs] [Bug 807] NEW: perform substitution when equality is known

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=807

           Summary: perform substitution when equality is known
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Scalar Optimizations
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: nicholas at mxc.ca


The following code doesn't optimize down:

// Source: pag.csail.mit.edu/paste2005/slides/yang-paste2005-slides.ppt

extern void foo();

void process(int *p, int *q)
{
  if (p != q)
    return;

  if (*p != *q)
    foo(); // unreachable
}

In a branch whose condition depends on a EQ or NE, in the side where the
variables are equal (and if the types are equal (?)), replaceAllUsesOfWith(p,
q). This allows the compiler to optimize away the following test and should kill
the liveness of one of the variables. I suppose you could also do the same
optimization with a switch instruction.

Where would such an optimization belong? The instruction combiner?



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