[SwiftError] Use IMPLICIT_DEF as a definition for unreachable VReg uses

SwiftErrorValueTracking creates vregs at swifterror use sites and then
connects it with appropriate definitions after instruction selection.
To propagate swifterror values SwiftErrorValueTracking::propagateVRegs
iterates over basic blocks in RPO, but some vregs previously created
at use sites may be located in blocks that became unreachable after
instruction selection. Because of that there will no definition for
such vregs and that may cause issues down the pipeline.

To ensure that all vregs created by the SwiftErrorValueTracking will
be defined propagateVRegs was updated to insert IMPLICIT_DEF at the
beginning of unreachable blocks containing swifterror uses.

Related issue: llvm#59751

Reviewed By: compnerd

Differential Revision: https://reviews.llvm.org/D141053

llvm/lib/CodeGen/SwiftErrorValueTracking.cpp

@@ -253,6 +253,25 @@ void SwiftErrorValueTracking::propagateVRegs() {
         setCurrentVReg(MBB, SwiftErrorVal, PHIVReg);
     }
   }
+
+  // Create implicit defs for upward uses from unreachable blocks
+  MachineRegisterInfo &MRI = MF->getRegInfo();
+  for (const auto &Use : VRegUpwardsUse) {
+    const MachineBasicBlock *UseBB = Use.first.first;
+    Register VReg = Use.second;
+    if (!MRI.def_begin(VReg).atEnd())
+      continue;
+
+#ifdef EXPENSIVE_CHECKS
+    assert(std::find(RPOT.begin(), RPOT.end(), UseBB) == RPOT.end() &&
+           "Reachable block has VReg upward use without definition.");
+#endif
+
+    MachineBasicBlock *UseBBMut = MF->getBlockNumbered(UseBB->getNumber());
+
+    BuildMI(*UseBBMut, UseBBMut->getFirstNonPHI(), DebugLoc(),
+            TII->get(TargetOpcode::IMPLICIT_DEF), VReg);
+  }
 }
 
 void SwiftErrorValueTracking::preassignVRegs(

llvm/test/CodeGen/AArch64/swift-error-unreachable-use.ll

@@ -0,0 +1,39 @@
+; RUN: llc -mtriple aarch64-unknown-windows-msvc %s -filetype asm -o - | FileCheck %s
+; RUN: llc -mtriple aarch64-unknown-linux-gnu %s -filetype asm -o - | FileCheck %s
+; RUN: llc -mtriple aarch64-apple-macosx %s -filetype asm -o - | FileCheck %s
+; Regression test for https://github.com/llvm/llvm-project/issues/59751
+
+define void @"func"(i32** swifterror %0) #0 {
+; CHECK-LABEL: func:
+; CHECK:       {{.*}}%bb.0:
+; CHECK-NEXT:    b {{\.?}}LBB0_2
+; CHECK-NEXT:  {{\.?}}LBB0_1:{{.*}}%thirtythree
+; CHECK-NEXT:  {{.*}}=>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    b {{\.?}}LBB0_1
+; CHECK-NEXT:  {{\.?}}LBB0_2:{{.*}}%thirtyeight
+; CHECK-NEXT:    b {{\.?}}LBB0_3
+; CHECK-NEXT:  {{\.?}}LBB0_3:{{.*}}%thirtythree.preheader
+; CHECK-NEXT:    b {{\.?}}LBB0_1
+  br label %thirtyeight
+
+five:
+  br label %UelOc2l.exit
+
+common.ret:
+  ret void
+
+UelOc2l.exit:
+  %a = getelementptr inbounds [754 x i8*], [754 x i8*]* undef, i32 undef, i32 undef
+  %b = load i8*, i8** %a, align 8
+  %c = bitcast i8* %b to void ()*
+  call void %c()
+  br label %common.ret
+
+thirtythree:
+  br i1 false, label %UelOc2l.exit, label %thirtythree
+
+thirtyeight:
+  br i1 undef, label %thirtyeight, label %thirtythree
+}
+
+attributes #0 = { noinline optnone }