Process PHI Nodes with zeros in CustomUnsafeOptimization pass

Process PHI Nodes with floating point zeros in CustomUnsafeOptimization pass.
Adding `FNeg` support to SetFastMathFlag pass.

Author: agrabezh

IGC/Compiler/CustomUnsafeOptPass.cpp

@@ -82,15 +82,9 @@ char CustomUnsafeOptPass::ID = 0;
 STATISTIC(Stat_FcmpRemoved, "Number of insts removed in FCmp Opt");
 STATISTIC(Stat_FloatRemoved, "Number of insts removed in Float Opt");
 
-static bool allowUnsafeMathOpt(CodeGenContext* ctx, llvm::BinaryOperator& op)
+static bool allowUnsafeMathOpt(CodeGenContext* ctx)
 {
-    // always allow unsafe opt if instruction has the flag
-    if (llvm::isa<llvm::FPMathOperator>(op) && op.getFastMathFlags().isFast())
-    {
-        return true;
-    }
-
-    // then check compiler options in metadata
+    // check compiler options in metadata
     if ((!ctx->m_checkFastFlagPerInstructionInCustomUnsafeOptPass &&
         ctx->getModuleMetaData()->compOpt.FastRelaxedMath) ||
         ctx->getModuleMetaData()->compOpt.UnsafeMathOptimizations)
@@ -106,6 +100,18 @@ static bool allowUnsafeMathOpt(CodeGenContext* ctx, llvm::BinaryOperator& op)
     return false;
 }
 
+static bool allowUnsafeMathOpt(CodeGenContext* ctx, llvm::BinaryOperator& op)
+{
+    // always allow unsafe opt if instruction has the flag
+    if (llvm::isa<llvm::FPMathOperator>(op) && op.getFastMathFlags().isFast())
+    {
+        return true;
+    }
+
+    // then check compiler options in metadata
+    return allowUnsafeMathOpt(ctx);
+}
+
 CustomUnsafeOptPass::CustomUnsafeOptPass()
     : FunctionPass(ID),
     m_disableReorderingOpt(0),
@@ -166,6 +172,31 @@ void CustomUnsafeOptPass::visitInstruction(Instruction& I)
     // nothing
 }
 
+void CustomUnsafeOptPass::visitPHINode(llvm::PHINode& I)
+{
+    /*
+    From
+        %zero = phi float [ -0.000000e+00, %if.then ], [ 0.000000e+00, %for.body ]
+        %mul = fmul fast float %zero, %smth
+    To
+        %mul = fmul fast float 0.0, %smth
+    */
+    if (I.getType()->isFloatingPointTy() && allowUnsafeMathOpt(m_ctx))
+    {
+        bool isZeroFP = llvm::all_of(I.incoming_values(), [](Use& use)
+        {
+            return isa<ConstantFP>(use.get()) && cast<ConstantFP>(use.get())->isZero();
+        });
+
+        if (isZeroFP)
+        {
+            I.replaceAllUsesWith(ConstantFP::get(I.getType(), 0.0));
+            I.eraseFromParent();
+            m_isChanged = true;
+        }
+    }
+}
+
 void CustomUnsafeOptPass::visitFPToSIInst(llvm::FPToSIInst& I)
 {
     /*

IGC/Compiler/CustomUnsafeOptPass.hpp

@@ -48,6 +48,7 @@ namespace IGC
         }
 
         void visitInstruction(llvm::Instruction& I);
+        void visitPHINode(llvm::PHINode& I);
         void visitBinaryOperator(llvm::BinaryOperator& I);
         void visitFCmpInst(llvm::FCmpInst& FC);
         void visitSelectInst(llvm::SelectInst& I);

IGC/Compiler/Optimizer/OpenCLPasses/SetFastMathFlags/SetFastMathFlags.cpp

@@ -121,6 +121,7 @@ bool SetFastMathFlags::setFlags(Function& F, FastMathFlags fmfs)
     {
         unsigned int op = i->getOpcode();
         switch (op) {
+        case Instruction::FNeg:
         case Instruction::FAdd:
         case Instruction::FSub:
         case Instruction::FMul:

IGC/Compiler/tests/CustomUnsafeOptPass/zero_phi.ll

@@ -0,0 +1,43 @@
+;=========================== begin_copyright_notice ============================
+;
+; Copyright (C) 2024 Intel Corporation
+;
+; SPDX-License-Identifier: MIT
+;
+;============================ end_copyright_notice =============================
+
+; RUN: igc_opt %s -S -o - -igc-custom-unsafe-opt-pass | FileCheck %s
+
+; tests CustomUnsafeOptPass::visitPHINode
+; Transformation:
+;    From
+;        %zero = phi float [ -0.000000e+00, %if.then ], [ 0.000000e+00, %for.body ]
+;        %op = fmul fast float %zero, %smth
+;    To
+;        %op = fmul fast float 0.0, %smth
+
+define float @test_phi(i1 %flag, float %in) {
+; CHECK-LABEL: @test_phi(
+; CHECK:  if.end:
+; CHECK-NOT:  %zero.phi = phi float
+; CHECK:  ret float %in
+entry:
+  br i1 %flag, label %if.then, label %if.end
+
+if.then:                                        ; preds = %entry
+  br label %if.end
+
+if.end:                                           ; preds = %if.then, %entry
+  %zero.phi = phi float [ -0.000000e+00, %if.then ], [ 0.000000e+00, %entry ]
+  %mul = fmul fast float %zero.phi, 0x3FF3333340000000
+  %add = fadd fast float %in, %mul
+  ret float %add
+}
+
+!IGCMetadata = !{!0}
+
+!0 = !{!"ModuleMD", !1}
+!1 = !{!"compOpt", !2, !3, !4}
+!2 = !{!"FastRelaxedMath", i1 true}
+!3 = !{!"UnsafeMathOptimizations", i1 false}
+!4 = !{!"disableCustomUnsafeOpts", i1 false}