[llvm] [SimplifyLibCalls] fdim constant fold (PR #109235)

Matt Arsenault via llvm-commits llvm-commits at lists.llvm.org
Tue Oct 8 14:02:10 PDT 2024


================
@@ -3109,6 +3109,42 @@ Value *LibCallSimplifier::optimizeRemquo(CallInst *CI, IRBuilderBase &B) {
   return ConstantFP::get(CI->getType(), Rem);
 }
 
+/// Constant folds fdim
+Value *LibCallSimplifier::optimizeFdim(CallInst *CI, IRBuilderBase &B) {
+  // Cannot perform the fold unless the call has attribute memory(none)
+  if (!CI->doesNotAccessMemory())
+    return nullptr;
+
+  const APFloat *X, *Y;
+  // Check if both values are constants
+  if (!match(CI->getArgOperand(0), m_APFloat(X)) ||
----------------
arsenm wrote:

No problem! 

You didn't directly implement this as constant folding. It's almost a general case replacement of the call with a short sequence. A pure constant fold would be done entirely with APFloat, rather than attempting to create intermediate instructions.

It's probably a good idea to leave the general case for a follow up PR, since there are cost considerations (more targets have maxnum-like operations), and last I checked maximum was poorly supported by most backends.

https://github.com/llvm/llvm-project/pull/109235


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