[Transforms][Utils][PromoteMem2Reg] Propagate nnan flag on par with the nsz flag

Following the change introduced by the PR llvm#83381, this patch extends
it with the same treatment of the nnan fast-math flag. This is to
address the performance drop caused by PR#83200 which prevented vital
InstCombine transformation due to the lack of relevant fast-math
flags.

The PromoteMem2Reg utility is used by the SROA pass, where Phi nodes
are being created. Proposed change allows propagation of the nnan
flag down to these Phi nodes.

Author: pawosm-arm

llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp

@@ -394,6 +394,9 @@ struct PromoteMem2Reg {
   /// Whether the function has the no-signed-zeros-fp-math attribute set.
   bool NoSignedZeros = false;
 
+  /// Whether the function has the no-nans-fp-math attribute set.
+  bool NoNaNs = false;
+
 public:
   PromoteMem2Reg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT,
                  AssumptionCache *AC)
@@ -752,6 +755,7 @@ void PromoteMem2Reg::run() {
   ForwardIDFCalculator IDF(DT);
 
   NoSignedZeros = F.getFnAttribute("no-signed-zeros-fp-math").getValueAsBool();
+  NoNaNs = F.getFnAttribute("no-nans-fp-math").getValueAsBool();
 
   for (unsigned AllocaNum = 0; AllocaNum != Allocas.size(); ++AllocaNum) {
     AllocaInst *AI = Allocas[AllocaNum];
@@ -1140,6 +1144,11 @@ void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred,
         if (isa<FPMathOperator>(APN) && NoSignedZeros)
           APN->setHasNoSignedZeros(true);
 
+        // This allows select instruction folding relevant to floating point
+        // reductions whose operand is a PHI.
+        if (isa<FPMathOperator>(APN) && NoNaNs)
+          APN->setHasNoNaNs(true);
+
         // The currently active variable for this block is now the PHI.
         IncomingVals[AllocaNo] = APN;
         AllocaATInfo[AllocaNo].updateForNewPhi(APN, DIB);

llvm/test/Transforms/SROA/propagate-fast-math-flags-on-phi.ll

@@ -77,3 +77,84 @@ return:                          ; preds = %entry,%if.then
   %retval = load double, ptr %x.addr
   ret double %retval
 }
+
+define double @phi_with_nnan(double %x) "no-nans-fp-math"="true" {
+; CHECK-LABEL: define double @phi_with_nnan(
+; CHECK-SAME: double [[X:%.*]]) #[[ATTR2:[0-9]+]] {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp olt double [[X]], 0.000000e+00
+; CHECK-NEXT:    br i1 [[CMP]], label [[IF_THEN:%.*]], label [[RETURN:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    [[FNEG:%.*]] = fneg double [[X]]
+; CHECK-NEXT:    br label [[RETURN]]
+; CHECK:       return:
+; CHECK-NEXT:    [[X_ADDR_0:%.*]] = phi nnan double [ [[FNEG]], [[IF_THEN]] ], [ undef, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    ret double [[X_ADDR_0]]
+;
+entry:
+  %x.addr = alloca double
+  %cmp = fcmp olt double %x, 0.0
+  br i1 %cmp, label %if.then, label %return
+
+if.then:                         ; preds = %entry
+  %fneg = fneg double %x
+  store double %fneg, ptr %x.addr
+  br label %return
+
+return:                          ; preds = %entry,%if.then
+  %retval = load double, ptr %x.addr
+  ret double %retval
+}
+
+define <2 x double> @vector_phi_with_nnan(<2 x double> %x, i1 %cmp, <2 x double> %a, <2 x double> %b) "no-nans-fp-math"="true" {
+; CHECK-LABEL: define <2 x double> @vector_phi_with_nnan(
+; CHECK-SAME: <2 x double> [[X:%.*]], i1 [[CMP:%.*]], <2 x double> [[A:%.*]], <2 x double> [[B:%.*]]) #[[ATTR2]] {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[CMP]], label [[IF_THEN:%.*]], label [[RETURN:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    br label [[RETURN]]
+; CHECK:       return:
+; CHECK-NEXT:    [[X_ADDR_0:%.*]] = phi nnan <2 x double> [ [[B]], [[IF_THEN]] ], [ [[A]], [[ENTRY:%.*]] ]
+; CHECK-NEXT:    ret <2 x double> [[X_ADDR_0]]
+;
+entry:
+  %x.addr = alloca <2 x double>
+  store <2 x double> %a, ptr %x.addr
+  br i1 %cmp, label %if.then, label %return
+
+if.then:                         ; preds = %entry
+  store <2 x double> %b, ptr %x.addr
+  br label %return
+
+return:                          ; preds = %entry,%if.then
+  %retval = load <2 x double>, ptr %x.addr
+  ret <2 x double> %retval
+}
+
+define double @phi_without_nnan(double %x) "no-nans-fp-math"="false" {
+; CHECK-LABEL: define double @phi_without_nnan(
+; CHECK-SAME: double [[X:%.*]]) #[[ATTR3:[0-9]+]] {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp olt double [[X]], 0.000000e+00
+; CHECK-NEXT:    br i1 [[CMP]], label [[IF_THEN:%.*]], label [[RETURN:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    [[FNEG:%.*]] = fneg double [[X]]
+; CHECK-NEXT:    br label [[RETURN]]
+; CHECK:       return:
+; CHECK-NEXT:    [[X_ADDR_0:%.*]] = phi double [ [[FNEG]], [[IF_THEN]] ], [ undef, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    ret double [[X_ADDR_0]]
+;
+entry:
+  %x.addr = alloca double
+  %cmp = fcmp olt double %x, 0.0
+  br i1 %cmp, label %if.then, label %return
+
+if.then:                         ; preds = %entry
+  %fneg = fneg double %x
+  store double %fneg, ptr %x.addr
+  br label %return
+
+return:                          ; preds = %entry,%if.then
+  %retval = load double, ptr %x.addr
+  ret double %retval
+}