[InstCombine] Make fptrunc combine use union of fast math flags

These combines involve swapping the fptrunc with its operand, and
using the union of fast math flags is the safest option as e.g. if we
have (fptrunc (fneg ninf x)) then (fneg ninf (fptrunc x)) will not be
correct as if x is a not within the range of the destination type the
result of (fptrunc x) will be inf.

Author: john-brawn-arm

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

@@ -1847,15 +1847,16 @@ Instruction *InstCombinerImpl::visitFPTrunc(FPTruncInst &FPT) {
   Value *X;
   Instruction *Op = dyn_cast<Instruction>(FPT.getOperand(0));
   if (Op && Op->hasOneUse()) {
-    // FIXME: The FMF should propagate from the fptrunc, not the source op.
     IRBuilder<>::FastMathFlagGuard FMFG(Builder);
+    FastMathFlags FMF = FPT.getFastMathFlags();
     if (isa<FPMathOperator>(Op))
-      Builder.setFastMathFlags(Op->getFastMathFlags());
+      FMF &= Op->getFastMathFlags();
+    Builder.setFastMathFlags(FMF);
 
     if (match(Op, m_FNeg(m_Value(X)))) {
       Value *InnerTrunc = Builder.CreateFPTrunc(X, Ty);
-
-      return UnaryOperator::CreateFNegFMF(InnerTrunc, Op);
+      Value *Neg = Builder.CreateFNeg(InnerTrunc);
+      return replaceInstUsesWith(FPT, Neg);
     }
 
     // If we are truncating a select that has an extended operand, we can

llvm/test/Transforms/InstCombine/fpcast.ll

@@ -29,6 +29,17 @@ define half @test3(float %a) {
   ret half %c
 }
 
+define half @test3_fast(float %a) {
+; CHECK-LABEL: @test3_fast(
+; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc float [[A:%.*]] to half
+; CHECK-NEXT:    [[C:%.*]] = call half @llvm.fabs.f16(half [[TMP1]])
+; CHECK-NEXT:    ret half [[C]]
+;
+  %b = call float @llvm.fabs.f32(float %a)
+  %c = fptrunc fast float %b to half
+  ret half %c
+}
+
 define half @fneg_fptrunc(float %a) {
 ; CHECK-LABEL: @fneg_fptrunc(
 ; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc float [[A:%.*]] to half
@@ -78,6 +89,28 @@ define half @test4-fast(float %a) {
 ; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc fast float [[A:%.*]] to half
 ; CHECK-NEXT:    [[C:%.*]] = fneg fast half [[TMP1]]
 ; CHECK-NEXT:    ret half [[C]]
+;
+  %b = fsub fast float -0.0, %a
+  %c = fptrunc fast float %b to half
+  ret half %c
+}
+
+define half @test4-mixed-fast-1(float %a) {
+; CHECK-LABEL: @test4-mixed-fast-1(
+; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc float [[A:%.*]] to half
+; CHECK-NEXT:    [[C:%.*]] = fneg half [[TMP1]]
+; CHECK-NEXT:    ret half [[C]]
+;
+  %b = fsub float -0.0, %a
+  %c = fptrunc fast float %b to half
+  ret half %c
+}
+
+define half @test4-mixed-fast-2(float %a) {
+; CHECK-LABEL: @test4-mixed-fast-2(
+; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc float [[A:%.*]] to half
+; CHECK-NEXT:    [[C:%.*]] = fneg half [[TMP1]]
+; CHECK-NEXT:    ret half [[C]]
 ;
   %b = fsub fast float -0.0, %a
   %c = fptrunc float %b to half
@@ -89,6 +122,28 @@ define half @test4_unary_fneg-fast(float %a) {
 ; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc fast float [[A:%.*]] to half
 ; CHECK-NEXT:    [[C:%.*]] = fneg fast half [[TMP1]]
 ; CHECK-NEXT:    ret half [[C]]
+;
+  %b = fneg fast float %a
+  %c = fptrunc fast float %b to half
+  ret half %c
+}
+
+define half @test4_unary_fneg-mixed-fast-1(float %a) {
+; CHECK-LABEL: @test4_unary_fneg-mixed-fast-1(
+; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc float [[A:%.*]] to half
+; CHECK-NEXT:    [[C:%.*]] = fneg half [[TMP1]]
+; CHECK-NEXT:    ret half [[C]]
+;
+  %b = fneg float %a
+  %c = fptrunc fast float %b to half
+  ret half %c
+}
+
+define half @test4_unary_fneg-mixed-fast-2(float %a) {
+; CHECK-LABEL: @test4_unary_fneg-mixed-fast-2(
+; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc float [[A:%.*]] to half
+; CHECK-NEXT:    [[C:%.*]] = fneg half [[TMP1]]
+; CHECK-NEXT:    ret half [[C]]
 ;
   %b = fneg fast float %a
   %c = fptrunc float %b to half

llvm/test/Transforms/InstCombine/fptrunc.ll

@@ -61,6 +61,18 @@ define float @fptrunc_select_true_val(float %x, double %y, i1 %cond) {
   ret float %r
 }
 
+define float @fptrunc_fast_select_true_val(float %x, double %y, i1 %cond) {
+; CHECK-LABEL: @fptrunc_fast_select_true_val(
+; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc fast double [[Y:%.*]] to float
+; CHECK-NEXT:    [[NARROW_SEL:%.*]] = select i1 [[COND:%.*]], float [[TMP1]], float [[X:%.*]]
+; CHECK-NEXT:    ret float [[NARROW_SEL]]
+;
+  %e = fpext float %x to double
+  %sel = select fast i1 %cond, double %y, double %e
+  %r = fptrunc fast double %sel to float
+  ret float %r
+}
+
 define <2 x float> @fptrunc_select_false_val(<2 x float> %x, <2 x double> %y, <2 x i1> %cond) {
 ; CHECK-LABEL: @fptrunc_select_false_val(
 ; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc <2 x double> [[Y:%.*]] to <2 x float>
@@ -73,6 +85,18 @@ define <2 x float> @fptrunc_select_false_val(<2 x float> %x, <2 x double> %y, <2
   ret <2 x float> %r
 }
 
+define <2 x float> @fptrunc_nnan_select_false_val(<2 x float> %x, <2 x double> %y, <2 x i1> %cond) {
+; CHECK-LABEL: @fptrunc_nnan_select_false_val(
+; CHECK-NEXT:    [[TMP1:%.*]] = fptrunc nnan <2 x double> [[Y:%.*]] to <2 x float>
+; CHECK-NEXT:    [[NARROW_SEL:%.*]] = select <2 x i1> [[COND:%.*]], <2 x float> [[X:%.*]], <2 x float> [[TMP1]]
+; CHECK-NEXT:    ret <2 x float> [[NARROW_SEL]]
+;
+  %e = fpext <2 x float> %x to <2 x double>
+  %sel = select nnan <2 x i1> %cond, <2 x double> %e, <2 x double> %y
+  %r = fptrunc nnan <2 x double> %sel to <2 x float>
+  ret <2 x float> %r
+}
+
 declare void @use(float)
 
 define half @fptrunc_select_true_val_extra_use(half %x, float %y, i1 %cond) {