[SPIR-V] Fix OpDecorate emission after vreg def. (#114426)

In SPIR-V, OpDecorate instructions are allowed to forward-declare a
virtual register. But while we are at the MIR level, we must comply with
stricter rules, meaning OpDecorate should be emited after, not before
the reg definition.
(In some cases, we defined those just before, switching to just after).

Related to #110652

---------

Signed-off-by: Nathan Gauër <[email protected]>

Author: Keenuts

llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp

@@ -990,13 +990,13 @@ bool SPIRVInstructionSelector::selectMemOperation(Register ResVReg,
     Register VarReg = MRI->createGenericVirtualRegister(LLT::scalar(64));
     GR.add(GV, GR.CurMF, VarReg);
 
-    buildOpDecorate(VarReg, I, TII, SPIRV::Decoration::Constant, {});
     BuildMI(*I.getParent(), I, I.getDebugLoc(), TII.get(SPIRV::OpVariable))
         .addDef(VarReg)
         .addUse(GR.getSPIRVTypeID(VarTy))
         .addImm(SPIRV::StorageClass::UniformConstant)
         .addUse(Const)
         .constrainAllUses(TII, TRI, RBI);
+    buildOpDecorate(VarReg, I, TII, SPIRV::Decoration::Constant, {});
     SPIRVType *SourceTy = GR.getOrCreateSPIRVPointerType(
         ValTy, I, TII, SPIRV::StorageClass::UniformConstant);
     SrcReg = MRI->createGenericVirtualRegister(LLT::scalar(64));

llvm/lib/Target/SPIRV/SPIRVPreLegalizer.cpp

@@ -829,7 +829,7 @@ static void insertSpirvDecorations(MachineFunction &MF, MachineIRBuilder MIB) {
     for (MachineInstr &MI : MBB) {
       if (!isSpvIntrinsic(MI, Intrinsic::spv_assign_decoration))
         continue;
-      MIB.setInsertPt(*MI.getParent(), MI);
+      MIB.setInsertPt(*MI.getParent(), MI.getNextNode());
       buildOpSpirvDecorations(MI.getOperand(1).getReg(), MIB,
                               MI.getOperand(2).getMetadata());
       ToErase.push_back(&MI);

llvm/test/CodeGen/SPIRV/decoration-order.ll

@@ -0,0 +1,15 @@
+; RUN: %if spirv-tools %{ llc -O0 -verify-machineinstrs -mtriple=spirv64-unknown-unknown %s -o - -filetype=obj | spirv-val %}
+; This test checks the OpDecorate MIR is generated after the associated
+; vreg definition in the case of an array size declared through this lowering.
+
+define spir_func i32 @foo() {
+entry:
+  %var = alloca i64
+  br label %block
+
+block:
+  call void @llvm.memset.p0.i64(ptr align 8 %var, i8 0, i64 24, i1 false)
+  ret i32 0
+}
+
+declare void @llvm.memset.p0.i64(ptr nocapture writeonly, i8, i64, i1 immarg)