do instruction selection for G_BITCAST early

Author: VyacheslavLevytskyy

llvm/lib/Target/SPIRV/SPIRVPreLegalizer.cpp

@@ -165,6 +165,57 @@ static MachineInstr *findAssignTypeInstr(Register Reg,
   return nullptr;
 }
 
+static void buildOpBitcast(SPIRVGlobalRegistry *GR, MachineIRBuilder &MIB,
+                           Register ResVReg, Register OpReg) {
+  SPIRVType *ResType = GR->getSPIRVTypeForVReg(ResVReg);
+  SPIRVType *OpType = GR->getSPIRVTypeForVReg(OpReg);
+  assert(ResType && OpType && "Operand types are expected");
+  if (!GR->isBitcastCompatible(ResType, OpType))
+    report_fatal_error("incompatible result and operand types in a bitcast");
+  MachineRegisterInfo *MRI = MIB.getMRI();
+  if (!MRI->getRegClassOrNull(ResVReg))
+    MRI->setRegClass(ResVReg, GR->getRegClass(ResType));
+  MIB.buildInstr(SPIRV::OpBitcast)
+      .addDef(ResVReg)
+      .addUse(GR->getSPIRVTypeID(ResType))
+      .addUse(OpReg);
+}
+
+// We do instruction selections early instead of calling MIB.buildBitcast()
+// generating the general op code G_BITCAST. When MachineVerifier validates
+// G_BITCAST we see a check of a kind: if Source Type is equal to Destination
+// Type then report error "bitcast must change the type". This doesn't take into
+// account the notion of a typed pointer that is important for SPIR-V where a
+// user may and should use bitcast between pointers with different pointee types
+// (https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#OpBitcast).
+// It's important for correct lowering in SPIR-V, because interpretation of the
+// data type is not left to instructions that utilize the pointer, but encoded
+// by the pointer declaration, and the SPIRV target can and must handle the
+// declaration and use of pointers that specify the type of data they point to.
+// It's not feasible to improve validation of G_BITCAST using just information
+// provided by low level types of source and destination. Therefore we don't
+// produce G_BITCAST as the general op code with semantics different from
+// OpBitcast, but rather lower to OpBitcast immediately. As for now, the only
+// difference would be that CombinerHelper couldn't transform known patterns
+// around G_BUILD_VECTOR. See discussion
+// in https://github.com/llvm/llvm-project/pull/110270 for even more context.
+static void selectOpBitcasts(MachineFunction &MF, SPIRVGlobalRegistry *GR,
+                             MachineIRBuilder MIB) {
+  SmallVector<MachineInstr *, 16> ToErase;
+  for (MachineBasicBlock &MBB : MF) {
+    for (MachineInstr &MI : MBB) {
+      if (MI.getOpcode() != TargetOpcode::G_BITCAST)
+        continue;
+      MIB.setInsertPt(*MI.getParent(), MI);
+      buildOpBitcast(GR, MIB, MI.getOperand(0).getReg(),
+                     MI.getOperand(1).getReg());
+      ToErase.push_back(&MI);
+    }
+  }
+  for (MachineInstr *MI : ToErase)
+    MI->eraseFromParent();
+}
+
 static void insertBitcasts(MachineFunction &MF, SPIRVGlobalRegistry *GR,
                            MachineIRBuilder MIB) {
   // Get access to information about available extensions
@@ -202,15 +253,6 @@ static void insertBitcasts(MachineFunction &MF, SPIRVGlobalRegistry *GR,
       } else {
         GR->assignSPIRVTypeToVReg(AssignedPtrType, Def, MF);
         MIB.buildBitcast(Def, Source);
-        // MachineVerifier requires that bitcast must change the type.
-        // Change AddressSpace if needed to hint that Def and Source points to
-        // different types: this doesn't change actual code generation.
-        LLT DefType = MRI->getType(Def);
-        if (DefType == MRI->getType(Source))
-          MRI->setType(Def,
-                       LLT::pointer((DefType.getAddressSpace() + 1) %
-                                        SPIRVSubtarget::MaxLegalAddressSpace,
-                                    GR->getPointerSize()));
       }
     }
   }
@@ -1007,6 +1049,7 @@ bool SPIRVPreLegalizer::runOnMachineFunction(MachineFunction &MF) {
   removeImplicitFallthroughs(MF, MIB);
   insertSpirvDecorations(MF, MIB);
   insertInlineAsm(MF, GR, ST, MIB);
+  selectOpBitcasts(MF, GR, MIB);
 
   return true;
 }

llvm/test/CodeGen/SPIRV/pointers/phi-valid-operand-types-rev.ll

@@ -1,7 +1,4 @@
-; The goal of the test case is to ensure that OpPhi is consistent with respect to operand types.
-; -verify-machineinstrs is not available due to mutually exclusive requirements for G_BITCAST and G_PHI.
-
-; RUN: llc -O0 -mtriple=spirv64-unknown-unknown %s -o - | FileCheck %s
+; RUN: llc -verify-machineinstrs -O0 -mtriple=spirv64-unknown-unknown %s -o - | FileCheck %s
 ; RUN: %if spirv-tools %{ llc -O0 -mtriple=spirv64-unknown-unknown %s -o - -filetype=obj | spirv-val %}
 
 ; CHECK: %[[#Char:]] = OpTypeInt 8 0

llvm/test/CodeGen/SPIRV/pointers/phi-valid-operand-types.ll

@@ -1,7 +1,4 @@
-; The goal of the test case is to ensure that OpPhi is consistent with respect to operand types.
-; -verify-machineinstrs is not available due to mutually exclusive requirements for G_BITCAST and G_PHI.
-
-; RUN: llc -O0 -mtriple=spirv64-unknown-unknown %s -o - | FileCheck %s
+; RUN: llc -verify-machineinstrs -O0 -mtriple=spirv64-unknown-unknown %s -o - | FileCheck %s
 ; RUN: %if spirv-tools %{ llc -O0 -mtriple=spirv64-unknown-unknown %s -o - -filetype=obj | spirv-val %}
 
 ; CHECK: %[[#Char:]] = OpTypeInt 8 0