[SROA] Propagate no-signed-zeros(nsz) fast-math flag on the phi node using function attribute

Its expected that the sequence return X > 0.0 ? X : -X, compiled with -Ofast, produces fabs intrinsic. However, at this point, LLVM is unable to do so.

The above sequence goes through the following transformation during the pass pipeline:

SROA pass generates the phi node. Here, it does not infer the fast-math flags on the phi node unlike clang frontend typically does.
Phi node eventually gets translated into select instruction.
Because of missing no-signed-zeros(nsz) fast-math flag on the select instruction, InstCombine pass fails to fold the sequence into fabs intrinsic.
This patch, as a part of SROA, tries to propagate nsz fast-math flag on the phi node using function attribute enabling this folding.

Co-authored-by: Sushant Gokhale <[email protected]>

Author: sushgokh

llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp

@@ -41,6 +41,7 @@
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/Module.h"
+#include "llvm/IR/Operator.h"
 #include "llvm/IR/Type.h"
 #include "llvm/IR/User.h"
 #include "llvm/Support/Casting.h"
@@ -1113,6 +1114,17 @@ void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred,
         for (unsigned i = 0; i != NumEdges; ++i)
           APN->addIncoming(IncomingVals[AllocaNo], Pred);
 
+        // For the  sequence `return X > 0.0 ? X : -X`, it is expected that this
+        // results in fabs intrinsic. However, without no-signed-zeros(nsz) flag
+        // on the phi node generated at this stage, fabs folding does not
+        // happen. So, we try to infer nsz flag from the function attributes to
+        // enable this fabs folding.
+        if (APN->isComplete() && isa<FPMathOperator>(APN) &&
+            BB->getParent()
+                ->getFnAttribute("no-signed-zeros-fp-math")
+                .getValueAsBool())
+          APN->setHasNoSignedZeros(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

@@ -0,0 +1,79 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt < %s -passes='sroa' -S | FileCheck %s
+define double @phi_with_nsz(double %x) "no-signed-zeros-fp-math"="true" {
+; CHECK-LABEL: define double @phi_with_nsz(
+; CHECK-SAME: double [[X:%.*]]) #[[ATTR0:[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 nsz 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_nsz(<2 x double> %x, i1 %cmp, <2 x double> %a, <2 x double> %b) "no-signed-zeros-fp-math"="true" {
+; CHECK-LABEL: define <2 x double> @vector_phi_with_nsz(
+; CHECK-SAME: <2 x double> [[X:%.*]], i1 [[CMP:%.*]], <2 x double> [[A:%.*]], <2 x double> [[B:%.*]]) #[[ATTR0]] {
+; 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 nsz <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_nsz(double %x) "no-signed-zeros-fp-math"="false" {
+; CHECK-LABEL: define double @phi_without_nsz(
+; CHECK-SAME: double [[X:%.*]]) #[[ATTR1:[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
+}