[PATCH] D129743: [GlobalISel] Change widenScalar of G_FCONSTANT to mutate into G_CONSTANT.

[Amara Emerson via Phabricator via llvm-commits]

aemerson created this revision.
aemerson added reviewers: arsenm, paquette, foad.
aemerson added a project: LLVM.
Herald added subscribers: hiraditya, rovka.
Herald added a project: All.
aemerson requested review of this revision.
Herald added a subscriber: wdng.

Widening a G_FCONSTANT by extending and then generating G_FPTRUNC doesn't produce the same result all the time. Instead, we can just transform it to a G_CONSTANT of the same bit pattern and truncate using a plain G_TRUNC instead.

Fixes https://github.com/llvm/llvm-project/issues/56454


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D129743

Files:
  llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
  llvm/test/CodeGen/AArch64/GlobalISel/legalize-constant.mir
  llvm/test/CodeGen/AArch64/GlobalISel/legalize-fp16-fconstant.mir


Index: llvm/test/CodeGen/AArch64/GlobalISel/legalize-fp16-fconstant.mir
===================================================================
--- llvm/test/CodeGen/AArch64/GlobalISel/legalize-fp16-fconstant.mir
+++ llvm/test/CodeGen/AArch64/GlobalISel/legalize-fp16-fconstant.mir
@@ -8,8 +8,7 @@
 body: |
   bb.0:
     ; NO-FP16-LABEL: name: fp16
-    ; NO-FP16: [[C:%[0-9]+]]:_(s32) = G_FCONSTANT float 0.000000e+00
-    ; NO-FP16-NEXT: %cst:_(s16) = G_FPTRUNC [[C]](s32)
+    ; NO-FP16: %cst:_(s16) = G_CONSTANT i16 0
     ; NO-FP16-NEXT: $h0 = COPY %cst(s16)
     ; NO-FP16-NEXT: RET_ReallyLR implicit $h0
     ; FP16-LABEL: name: fp16
Index: llvm/test/CodeGen/AArch64/GlobalISel/legalize-constant.mir
===================================================================
--- llvm/test/CodeGen/AArch64/GlobalISel/legalize-constant.mir
+++ llvm/test/CodeGen/AArch64/GlobalISel/legalize-constant.mir
@@ -48,10 +48,8 @@
     ; CHECK-NEXT: $w0 = COPY [[C]](s32)
     ; CHECK-NEXT: [[C1:%[0-9]+]]:_(s64) = G_FCONSTANT double 2.000000e+00
     ; CHECK-NEXT: $x0 = COPY [[C1]](s64)
-    ; CHECK-NEXT: [[C2:%[0-9]+]]:_(s32) = G_FCONSTANT float 0.000000e+00
-    ; CHECK-NEXT: [[FPTRUNC:%[0-9]+]]:_(s16) = G_FPTRUNC [[C2]](s32)
-    ; CHECK-NEXT: [[ANYEXT:%[0-9]+]]:_(s32) = G_ANYEXT [[FPTRUNC]](s16)
-    ; CHECK-NEXT: $w0 = COPY [[ANYEXT]](s32)
+    ; CHECK-NEXT: [[C2:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
+    ; CHECK-NEXT: $w0 = COPY [[C2]](s32)
     %0:_(s32) = G_FCONSTANT float 1.0
     $w0 = COPY %0
     %1:_(s64) = G_FCONSTANT double 2.0
Index: llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -2393,30 +2393,14 @@
     return Legalized;
   }
   case TargetOpcode::G_FCONSTANT: {
+    // To avoid changing the bits of the constant due to extension to a larger
+    // type and then using G_FPTRUNC, we simply convert to a G_CONSTANT.
     MachineOperand &SrcMO = MI.getOperand(1);
-    LLVMContext &Ctx = MIRBuilder.getMF().getFunction().getContext();
-    APFloat Val = SrcMO.getFPImm()->getValueAPF();
-    bool LosesInfo;
-    switch (WideTy.getSizeInBits()) {
-    case 32:
-      Val.convert(APFloat::IEEEsingle(), APFloat::rmNearestTiesToEven,
-                  &LosesInfo);
-      break;
-    case 64:
-      Val.convert(APFloat::IEEEdouble(), APFloat::rmNearestTiesToEven,
-                  &LosesInfo);
-      break;
-    default:
-      return UnableToLegalize;
-    }
-
-    assert(!LosesInfo && "extend should always be lossless");
-
-    Observer.changingInstr(MI);
-    SrcMO.setFPImm(ConstantFP::get(Ctx, Val));
-
-    widenScalarDst(MI, WideTy, 0, TargetOpcode::G_FPTRUNC);
-    Observer.changedInstr(MI);
+    APInt Val = SrcMO.getFPImm()->getValueAPF().bitcastToAPInt();
+    MIRBuilder.setInstrAndDebugLoc(MI);
+    auto IntCst = MIRBuilder.buildConstant(MI.getOperand(0).getReg(), Val);
+    widenScalarDst(*IntCst, WideTy, 0, TargetOpcode::G_TRUNC);
+    MI.eraseFromParent();
     return Legalized;
   }
   case TargetOpcode::G_IMPLICIT_DEF: {


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