[AMDGPU] Avoid an illegal operand in si-shrink-instructions

[piotr]

Summary

Before the patch it was possible to trigger a constant bus
violation when folding immediates into a shrunk instruction.

The patch adds a check to enforce the legality of the new operand.

llvm/lib/Target/AMDGPU/SIShrinkInstructions.cpp

static bool foldImmediates(MachineInstr &MI, const SIInstrInfo *TII,
   if (Src0.isReg()) {
     Register Reg = Src0.getReg();
     if (Reg.isVirtual() && MRI.hasOneUse(Reg)) {
       MachineInstr *Def = MRI.getUniqueVRegDef(Reg);
       if (Def && Def->isMoveImmediate()) {
         MachineOperand &MovSrc = Def->getOperand(1);
         bool ConstantFolded = false;
 
-        if (MovSrc.isImm() && (isInt<32>(MovSrc.getImm()) ||
-                               isUInt<32>(MovSrc.getImm()))) {
+        if (TII->isOperandLegal(MI, Src0Idx, &MovSrc)) {
+          if (MovSrc.isImm() &&
+              (isInt<32>(MovSrc.getImm()) || isUInt<32>(MovSrc.getImm()))) {
             Src0.ChangeToImmediate(MovSrc.getImm());
             ConstantFolded = true;
           } else if (MovSrc.isFI()) {
             Src0.ChangeToFrameIndex(MovSrc.getIndex());
             ConstantFolded = true;
           } else if (MovSrc.isGlobal()) {
             Src0.ChangeToGA(MovSrc.getGlobal(), MovSrc.getOffset(),
                             MovSrc.getTargetFlags());
             ConstantFolded = true;
           }
+        }
 
         if (ConstantFolded) {
           assert(MRI.use_empty(Reg));
           Def->eraseFromParent();
           ++NumLiteralConstantsFolded;
           return true;
         }
       }

llvm/test/CodeGen/AMDGPU/shrink-instructions-illegal-fold.mir

+# RUN: llc -march=amdgcn -mcpu=gfx900 -run-pass=si-shrink-instructions --verify-machineinstrs %s -o - | FileCheck %s
+
+# Make sure immediate folding into V_CNDMASK respects constant bus restrictions.
+---
+
+name:            shrink_cndmask_illegal_imm_folding
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $vgpr0, $vgpr1
+    ; CHECK-LABEL: name: shrink_cndmask_illegal_imm_folding
+    ; CHECK: [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+    ; CHECK: [[MOV:%[0-9]+]]:vgpr_32 = V_MOV_B32_e32 32768, implicit $exec
+    ; CHECK: V_CMP_EQ_U32_e32 0, [[COPY]], implicit-def $vcc, implicit $exec
+    ; CHECK: V_CNDMASK_B32_e32 [[MOV]], killed [[COPY]], implicit $vcc, implicit $exec
+
+    %0:vgpr_32 = COPY $vgpr0
+    %1:vgpr_32 = V_MOV_B32_e32 32768, implicit $exec
+    V_CMP_EQ_U32_e32 0, %0:vgpr_32, implicit-def $vcc, implicit $exec
+    %2:vgpr_32 = V_CNDMASK_B32_e64 0, %1:vgpr_32, 0, killed %0:vgpr_32, $vcc, implicit $exec
+    S_NOP 0
+
+...