[PATCH] D141053: [SwiftError] Use IMPLICIT_DEF as a definition for unreachable VReg uses

[Filipp Zhinkin via Phabricator via llvm-commits]

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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: https://github.com/llvm/llvm-project/issues/59751


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D141053

Files:
  llvm/lib/CodeGen/SwiftErrorValueTracking.cpp
  llvm/test/CodeGen/AArch64/swift-error-unreachable-use.ll


Index: llvm/test/CodeGen/AArch64/swift-error-unreachable-use.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AArch64/swift-error-unreachable-use.ll
@@ -0,0 +1,30 @@
+; RUN: llc < %s
+;
+; Regression test for https://github.com/llvm/llvm-project/issues/59751
+
+target triple = "aarch64-unknown"
+
+define void @"func"(i32** swifterror %0) #0 {
+  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 }
\ No newline at end of file
Index: llvm/lib/CodeGen/SwiftErrorValueTracking.cpp
===================================================================
--- llvm/lib/CodeGen/SwiftErrorValueTracking.cpp
+++ llvm/lib/CodeGen/SwiftErrorValueTracking.cpp
@@ -253,6 +253,24 @@
         setCurrentVReg(MBB, SwiftErrorVal, PHIVReg);
     }
   }
+
+  // Create implicit defs for upward uses from unreachable blocks
+  MachineRegisterInfo &MRI = MF->getRegInfo();
+  for (auto UseIt = VRegUpwardsUse.begin(), End = VRegUpwardsUse.end();
+       UseIt != End; ++UseIt) {
+    const MachineBasicBlock *UseBB = UseIt->first.first;
+    Register VReg = UseIt->second;
+    if (!MRI.def_begin(VReg).atEnd())
+      continue;
+
+    assert(std::find(RPOT.begin(), RPOT.end(), UseBB) == RPOT.end() &&
+           "Reachable block has VReg upward use without definition.");
+
+    MachineBasicBlock *UseBBMut = MF->getBlockNumbered(UseBB->getNumber());
+
+    BuildMI(*UseBBMut, UseBBMut->getFirstNonPHI(), DebugLoc(),
+            TII->get(TargetOpcode::IMPLICIT_DEF), VReg);
+  }
 }
 
 void SwiftErrorValueTracking::preassignVRegs(


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