[llvm] d4f84df - [GlobalISel] Change widenScalar of G_FCONSTANT to mutate into G_CONSTANT.

[Amara Emerson via llvm-commits]

Author: Amara Emerson
Date: 2022-07-14T11:05:10-07:00
New Revision: d4f84df0a008b13d0e8440aec1d63651b7520923

URL: https://github.com/llvm/llvm-project/commit/d4f84df0a008b13d0e8440aec1d63651b7520923
DIFF: https://github.com/llvm/llvm-project/commit/d4f84df0a008b13d0e8440aec1d63651b7520923.diff

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

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

Differential Revision: https://reviews.llvm.org/D129743

Added: 
    

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

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index fb046d519ac89..52ee13757f276 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -2393,30 +2393,14 @@ LegalizerHelper::widenScalar(MachineInstr &MI, unsigned TypeIdx, LLT WideTy) {
     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: {

diff  --git a/llvm/test/CodeGen/AArch64/GlobalISel/legalize-constant.mir b/llvm/test/CodeGen/AArch64/GlobalISel/legalize-constant.mir
index 07eec1101e0a3..96be30b31b627 100644
--- a/llvm/test/CodeGen/AArch64/GlobalISel/legalize-constant.mir
+++ b/llvm/test/CodeGen/AArch64/GlobalISel/legalize-constant.mir
@@ -48,10 +48,8 @@ body: |
     ; 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

diff  --git a/llvm/test/CodeGen/AArch64/GlobalISel/legalize-fp16-fconstant.mir b/llvm/test/CodeGen/AArch64/GlobalISel/legalize-fp16-fconstant.mir
index 4d4a3693fe316..44d6b95eb5491 100644
--- a/llvm/test/CodeGen/AArch64/GlobalISel/legalize-fp16-fconstant.mir
+++ b/llvm/test/CodeGen/AArch64/GlobalISel/legalize-fp16-fconstant.mir
@@ -8,8 +8,7 @@ tracksRegLiveness: true
 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
@@ -19,4 +18,42 @@ body: |
     %cst:_(s16) = G_FCONSTANT half 0.0
     $h0 = COPY %cst
     RET_ReallyLR implicit $h0
+...
+---
+name:            fp16_non_zero
+tracksRegLiveness: true
+body: |
+  bb.0:
+    ; NO-FP16-LABEL: name: fp16_non_zero
+    ; NO-FP16: %cst:_(s16) = G_CONSTANT i16 16384
+    ; NO-FP16-NEXT: $h0 = COPY %cst(s16)
+    ; NO-FP16-NEXT: RET_ReallyLR implicit $h0
+    ; FP16-LABEL: name: fp16_non_zero
+    ; FP16: %cst:_(s16) = G_FCONSTANT half 0xH4000
+    ; FP16-NEXT: $h0 = COPY %cst(s16)
+    ; FP16-NEXT: RET_ReallyLR implicit $h0
+    %cst:_(s16) = G_FCONSTANT half 2.0
+    $h0 = COPY %cst
+    RET_ReallyLR implicit $h0
+...
+---
+name:            nan
+tracksRegLiveness: true
+body:             |
+  bb.1.entry:
+    ; NO-FP16-LABEL: name: nan
+    ; NO-FP16: %cst:_(s16) = G_CONSTANT i16 31745
+    ; NO-FP16-NEXT: %ext:_(s32) = G_FPEXT %cst(s16)
+    ; NO-FP16-NEXT: $w0 = COPY %ext(s32)
+    ; NO-FP16-NEXT: RET_ReallyLR implicit $w0
+    ; FP16-LABEL: name: nan
+    ; FP16: %cst:_(s16) = G_FCONSTANT half 0xH7C01
+    ; FP16-NEXT: %ext:_(s32) = G_FPEXT %cst(s16)
+    ; FP16-NEXT: $w0 = COPY %ext(s32)
+    ; FP16-NEXT: RET_ReallyLR implicit $w0
+    %cst:_(s16) = G_FCONSTANT half 0xH7C01
+    %ext:_(s32) = G_FPEXT %cst(s16)
+    $w0 = COPY %ext(s32)
+    RET_ReallyLR implicit $w0
 
+...