Fix for lost FastMathFlags in GVN

[v_klochkov]

Summary

Hello,

Please review the fix for lost fast-math flags in GVN (Global Value Numbering) optimization.

Thank you,
Vyacheslav Klochkov

[v_klochkov]

I just tightened the checks in the LIT test.

[hfinkel]

LGTM

llvm/lib/Transforms/Scalar/GVN.cpp

 
 static void patchReplacementInstruction(Instruction *I, Value *Repl) {
   auto *ReplInst = dyn_cast<Instruction>(Repl);
   if (!ReplInst)
     return;
 
   // Patch the replacement so that it is not more restrictive than the value
   // being replaced.
+  // Note that if 'I' is a load being replaced by some operation, 
+  // for example, by an arithmetic operation, then andIRFlags()
+  // would just erase all math flags from the original arithmetic
+  // operation, which is clearly not wanted and not needed.
+  if (!isa<LoadInst>(I))
     ReplInst->andIRFlags(I);
 
   // FIXME: If both the original and replacement value are part of the
   // same control-flow region (meaning that the execution of one
   // guarantees the execution of the other), then we can combine the
   // noalias scopes here and do better than the general conservative
   // answer used in combineMetadata().
 
   // In general, GVN unifies expressions over different control-flow

llvm/test/Transforms/GVN/propagate-ir-flags.ll

+
+; RUN: opt < %s -gvn -S | FileCheck %s
+
+; CHECK-LABEL: @func_fast(
+; CHECK: fadd fast double
+define double @func_fast(double %a, double %b) {
+entry:
+  %a.addr = alloca double, align 8
+  %add = fadd fast double %b, 3.000000e+00
+  store double %add, double* %a.addr, align 8
+  %load_add = load double, double* %a.addr, align 8
+  ret double %load_add
+}
+
+; CHECK-LABEL: @func_no_fast(
+; CHECK: fadd double
+define double @func_no_fast(double %a, double %b) {
+entry:
+  %a.addr = alloca double, align 8
+  %add = fadd fast double %b, 3.000000e+00
+  store double %add, double* %a.addr, align 8
+  %duplicated_add = fadd double %b, 3.000000e+00
+  ret double %duplicated_add
+}
+