[LLVMbugs] [Bug 537] NEW: [instcombine] ptrdiff_t not optimized as well as it could be

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.cs.uiuc.edu/bugs/show_bug.cgi?id=537

           Summary: [instcombine] ptrdiff_t not optimized as well as it
                    could be
           Product: libraries
           Version: 1.0
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Scalar Optimizations
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: sabre at nondot.org


Markus Oberhumer writes:

In the example below llvm is only able to optimize
away the subtraction in test1, but not in test2/3.


#include <stddef.h>

int test1(int a, int b) {
    int d = b - a;
    a = b - d;          // no-op
    return a;
}

char* test2(char* a, char* b) {
    ptrdiff_t d = b - a;
    a = b - d;          // no-op on all llvm-supported machines
    return a;
}

int test3(char* a, char* b, char* c) {
    ptrdiff_t d = b - a;
    return (d == 8) || ((a = b - d) < c);    // a = ... is a no-op
}



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