[RegAllocFast] insert additional spills along indirect edges of INLINEASM_BR

When generating spills (stores) for values produced by INLINEASM_BR
instructions, make sure to insert one spill per indirect target.
Otherwise the reload generated may load from a stack slot that has not
yet been stored to (resulting in a load of an uninitialized stack slot).

Link: #53562
Fixes: #60855

Reviewed By: MatzeB

Differential Revision: https://reviews.llvm.org/D144907

Author: nickdesaulniers

llvm/lib/CodeGen/RegAllocFast.cpp

@@ -948,6 +948,23 @@ void RegAllocFast::defineVirtReg(MachineInstr &MI, unsigned OpNum,
                         << LRI->Reloaded << '\n');
       bool Kill = LRI->LastUse == nullptr;
       spill(SpillBefore, VirtReg, PhysReg, Kill, LRI->LiveOut);
+
+      // We need to place additional spills for each indirect destination of an
+      // INLINEASM_BR.
+      if (MI.getOpcode() == TargetOpcode::INLINEASM_BR) {
+        int FI = StackSlotForVirtReg[VirtReg];
+        const TargetRegisterClass &RC = *MRI->getRegClass(VirtReg);
+        for (MachineOperand &MO : MI.operands()) {
+          if (MO.isMBB()) {
+            MachineBasicBlock *Succ = MO.getMBB();
+            TII->storeRegToStackSlot(*Succ, Succ->begin(), PhysReg, Kill,
+                FI, &RC, TRI, VirtReg);
+            ++NumStores;
+            Succ->addLiveIn(PhysReg);
+          }
+        }
+      }
+
       LRI->LastUse = nullptr;
     }
     LRI->LiveOut = false;

llvm/test/CodeGen/X86/callbr-asm-outputs-regallocfast.mir

@@ -135,8 +135,9 @@ body:             |
   ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT: bb.3.label.split (machine-block-address-taken, inlineasm-br-indirect-target):
   ; CHECK-NEXT:   successors: %bb.2(0x80000000)
+  ; CHECK-NEXT:   liveins: $eax
   ; CHECK-NEXT: {{  $}}
-  ; FIXME: this is a load from a stack slot that hasn't been stored to yet!
+  ; CHECK-NEXT:   MOV32mr %stack.3, 1, $noreg, 0, $noreg, $eax :: (store (s32) into %stack.3)
   ; CHECK-NEXT:   $eax = MOV32rm %stack.3, 1, $noreg, 0, $noreg :: (load (s32) from %stack.3)
   ; CHECK-NEXT:   MOV32mr %stack.1.x, 1, $noreg, 0, $noreg, killed renamable $eax :: (store (s32) into %ir.x)
   ; CHECK-NEXT:   JMP_1 %bb.2