[LLVMdev] eliminating redundant pointer arithmethics

[Chris Lattner]

On Fri, 11 Mar 2005, Markus F.X.J. Oberhumer wrote:
> Is there an optimization pass that eliminates redundant pointer
> arithmethics ? In the example below llvm is only able to optimize
> away the subtraction in test1, but not in test2/3.

Nope, there isn't anything yet.  This should be handled by the instruction 
combiner.  I'll file an enhancement PR for this, and sometime when I have 
a couple of spare cycles I'll implement it (unless someone beats me to it 
:)

Thanks for noticing this!

-Chris

> #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
> }
>
>
>

-Chris

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