Add support for the 'freeze' instruction

[VyacheslavLevytskyy]

This PR is to add support for the 'freeze' instruction: https://llvm.org/docs/LangRef.html#freeze-instruction

There is no way to implement freeze correctly without support on SPIR-V standard side, but we may at least address a simple (static) case when undef/poison value presence is obvious. The main benefit of even incomplete freeze support is preventing of translation from crashing due to lack of support on legalization and instruction selection steps.

[llvmbot]

@llvm/pr-subscribers-backend-spir-v

Author: Vyacheslav Levytskyy (VyacheslavLevytskyy)

Changes

This PR is to add support for the 'freeze' instruction: https://llvm.org/docs/LangRef.html#freeze-instruction

There is no way to implement freeze correctly without support on SPIR-V standard side, but we may at least address a simple (static) case when undef/poison value presence is obvious. The main benefit of even incomplete freeze support is preventing of translation from crashing due to lack of support on legalization and instruction selection steps.

---

Full diff: https://github.com/llvm/llvm-project/pull/82979.diff

3 Files Affected:

• (modified) llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp (+44)
• (modified) llvm/lib/Target/SPIRV/SPIRVLegalizerInfo.cpp (+1-1)
• (added) llvm/test/CodeGen/SPIRV/freeze.ll (+37)

diff --git a/llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp b/llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp
index 7258d3b4d88ed3..f1e18f0601b87c 100644
--- a/llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp
+++ b/llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp
@@ -150,6 +150,8 @@ class SPIRVInstructionSelector : public InstructionSelector {
 
   bool selectOpUndef(Register ResVReg, const SPIRVType *ResType,
                      MachineInstr &I) const;
+  bool selectFreeze(Register ResVReg, const SPIRVType *ResType,
+                    MachineInstr &I) const;
   bool selectIntrinsic(Register ResVReg, const SPIRVType *ResType,
                        MachineInstr &I) const;
   bool selectExtractVal(Register ResVReg, const SPIRVType *ResType,
@@ -284,6 +286,8 @@ bool SPIRVInstructionSelector::spvSelect(Register ResVReg,
     return selectGlobalValue(ResVReg, I);
   case TargetOpcode::G_IMPLICIT_DEF:
     return selectOpUndef(ResVReg, ResType, I);
+  case TargetOpcode::G_FREEZE:
+    return selectFreeze(ResVReg, ResType, I);
 
   case TargetOpcode::G_INTRINSIC_W_SIDE_EFFECTS:
   case TargetOpcode::G_INTRINSIC_CONVERGENT_W_SIDE_EFFECTS:
@@ -1014,6 +1018,46 @@ bool SPIRVInstructionSelector::selectBitreverse(Register ResVReg,
       .constrainAllUses(TII, TRI, RBI);
 }
 
+bool SPIRVInstructionSelector::selectFreeze(Register ResVReg,
+                                            const SPIRVType *ResType,
+                                            MachineInstr &I) const {
+  // There is no way to implement `freeze` correctly without support on SPIR-V
+  // standard side, but we may at least address a simple (static) case when
+  // undef/poison value presence is obvious. The main benefit of even
+  // incomplete `freeze` support is preventing of translation from crashing due
+  // to lack of support on legalization and instruction selection steps.
+  if (!I.getOperand(0).isReg() || !I.getOperand(1).isReg())
+    return false;
+  Register OpReg = I.getOperand(1).getReg();
+  if (MachineInstr *Def = MRI->getVRegDef(OpReg)) {
+    Register Reg;
+    switch (Def->getOpcode()) {
+    case SPIRV::ASSIGN_TYPE:
+      if (MachineInstr *AssignToDef =
+              MRI->getVRegDef(Def->getOperand(1).getReg())) {
+        if (AssignToDef->getOpcode() == TargetOpcode::G_IMPLICIT_DEF)
+          Reg = Def->getOperand(2).getReg();
+      }
+      break;
+    case SPIRV::OpUndef:
+      Reg = Def->getOperand(1).getReg();
+      break;
+    }
+    unsigned DestOpCode;
+    if (Reg.isValid()) {
+      DestOpCode = SPIRV::OpConstantNull;
+    } else {
+      DestOpCode = TargetOpcode::COPY;
+      Reg = OpReg;
+    }
+    return BuildMI(*I.getParent(), I, I.getDebugLoc(), TII.get(DestOpCode))
+        .addDef(I.getOperand(0).getReg())
+        .addUse(Reg)
+        .constrainAllUses(TII, TRI, RBI);
+  }
+  return false;
+}
+
 bool SPIRVInstructionSelector::selectConstVector(Register ResVReg,
                                                  const SPIRVType *ResType,
                                                  MachineInstr &I) const {
diff --git a/llvm/lib/Target/SPIRV/SPIRVLegalizerInfo.cpp b/llvm/lib/Target/SPIRV/SPIRVLegalizerInfo.cpp
index 4f2e7a240fc2cc..aedca798b4f6ae 100644
--- a/llvm/lib/Target/SPIRV/SPIRVLegalizerInfo.cpp
+++ b/llvm/lib/Target/SPIRV/SPIRVLegalizerInfo.cpp
@@ -184,7 +184,7 @@ SPIRVLegalizerInfo::SPIRVLegalizerInfo(const SPIRVSubtarget &ST) {
         return Query.Types[0].getSizeInBits() == Query.Types[1].getSizeInBits();
       }))));
 
-  getActionDefinitionsBuilder(G_IMPLICIT_DEF).alwaysLegal();
+  getActionDefinitionsBuilder({G_IMPLICIT_DEF, G_FREEZE}).alwaysLegal();
 
   getActionDefinitionsBuilder(G_INTTOPTR)
       .legalForCartesianProduct(allPtrs, allIntScalars);
diff --git a/llvm/test/CodeGen/SPIRV/freeze.ll b/llvm/test/CodeGen/SPIRV/freeze.ll
new file mode 100644
index 00000000000000..fe4335722fe8b8
--- /dev/null
+++ b/llvm/test/CodeGen/SPIRV/freeze.ll
@@ -0,0 +1,37 @@
+; RUN: llc -O0 -mtriple=spirv32-unknown-unknown %s -o - | FileCheck %s
+; TODO: %if spirv-tools %{ llc -O0 -mtriple=spirv32-unknown-unknown %s -o - -filetype=obj | spirv-val %}
+
+; CHECK: OpName %[[Arg1:.*]] "arg1"
+; CHECK: OpName %[[Arg2:.*]] "arg2"
+; CHECK: OpName %[[NotAStaticPoison:.*]] "poison1"
+; CHECK: OpName %[[NotAStaticPoison]] "nil0"
+; CHECK: OpName %[[StaticPoisonIntFreeze:.*]] "nil1"
+; CHECK: OpName %[[StaticPoisonFloatFreeze:.*]] "nil2"
+; CHECK: OpName %[[Arg1]] "val1"
+; CHECK: OpName %[[Const100:.*]] "val2"
+; CHECK: OpName %[[Const100]] "val3"
+; CHECK: OpDecorate
+; CHECK-DAG: %[[FloatTy:.*]] = OpTypeFloat 32
+; CHECK-DAG: %[[ShortTy:.*]] = OpTypeInt 16 0
+; CHECK-DAG: %[[IntTy:.*]] = OpTypeInt 32 0
+; CHECK-DAG: %[[Undef:.*]] = OpUndef %[[ShortTy]]
+; CHECK-DAG: %[[Const100]] = OpConstant %[[IntTy]] 100
+; CHECK-DAG: %[[StaticPoisonIntFreeze]] = OpConstantNull %[[IntTy]]
+; CHECK-DAG: %[[StaticPoisonFloatFreeze]] = OpConstantNull %[[FloatTy]]
+; CHECK: %[[Arg1]] = OpFunctionParameter %[[FloatTy]]
+; CHECK: %[[NotAStaticPoison]] = OpIAdd %[[ShortTy]] %[[Arg2]] %[[Undef]]
+
+target datalayout = "e-i64:64-v16:16-v24:32-v32:32-v48:64-v96:128-v192:256-v256:256-v512:512-v1024:1024-n8:16:32:64"
+target triple = "spir64-unknown-unknown"
+
+define spir_func void @foo(float %arg1, i16 %arg2) {
+entry:
+  %poison1 = add i16 %arg2, undef
+  %nil0 = freeze i16 %poison1
+  %nil1 = freeze i32 undef
+  %nil2 = freeze float poison
+  %val1 = freeze float %arg1
+  %val2 = freeze i32 100
+  %val3 = freeze i32 %val2
+  ret void
+}