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Hi,</div>
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I hope it's a right place to ask. I'm looking for a specific compiler transformation, that I'm not sure it exists, and I hoped someone could point me in the right direction.</div>
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I think the easiest way to explain it is by using an example:</div>
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Example (1):<br>
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void zero(int64_t A, int32_t N) {
<div> for(int32_t i = 0; i < N; i++) {</div>
<div> ((int*)A)[i] = 0;</div>
<div> }</div>
}</div>
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Example (2):</div>
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void zero(int64_t A, int32_t N) {
<div> for(int32_t i = 0; i < N; i++) {</div>
<div> *(int*)(A + i * 4) = 0;</div>
<div> }</div>
}</div>
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In principle both snippets do the same thing, they set zero to N elements of an array A (passed as an address of A). In (1) the compiler (armv8-a clang 11.0.1) can detect that the operation is a memset, and when compiled with -O3 it indeed emits a call to memset.
However, the compiler cannot optimize code in (2).<br>
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The big difference in emitted LLVM IR (when no optimizations are enabled) is that in (1) the compiler emits inttoptr to cast the address of A followed by getelementptr. Whereas for (2) the address of the array element is first calculated using basic integer
operations (mul and add) and only then casted to pointer using inttoptr. I believe that the compiler can optimize (1) as getelementptr provides certain guarantees that enable safe optimizations, but simple cast of the element address with inttoptr is too broad.</div>
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So, the question is if there is a transformation (pointers to proof-of-concept academic impelemntation would be equally appreciated) in LLVM toolchain that can transform problem (2) into (1), so it can be further optimized. I understand this may be a complex
problem in the genral case, so asking for such transformation may sound a bit naive, but I thought I'd try.</div>
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Many thanks,</div>
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Igor Wodiany<br>
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