[DebugInfo][RemoveDIs] Don't convert debug-intrinsics to Unreachable (#72380)

It might seem obvious, but it's not a good idea to convert a
debug-intrinsic instruction into an UnreachableInst, as this means
things operate differently with and without the -g option. However this
can happen due to the "mutate the next instruction" API calls we make.
With RemoveDIs eliminating debug intrinsics, this behaviour is at risk
of changing, hence this patch ensures we only ever mutate the next _non_
debuginfo instruction into an Unreachable.

The tests instrumented with the --try... flag all exercise this, I've
added some metadata to a SCCP test to ensure it's exercised.

Author: jmorse

llvm/lib/Transforms/IPO/SCCP.cpp

@@ -236,11 +236,11 @@ static bool runIPSCCP(
     // nodes in executable blocks we found values for. The function's entry
     // block is not part of BlocksToErase, so we have to handle it separately.
     for (BasicBlock *BB : BlocksToErase) {
-      NumInstRemoved += changeToUnreachable(BB->getFirstNonPHI(),
+      NumInstRemoved += changeToUnreachable(BB->getFirstNonPHIOrDbg(),
                                             /*PreserveLCSSA=*/false, &DTU);
     }
     if (!Solver.isBlockExecutable(&F.front()))
-      NumInstRemoved += changeToUnreachable(F.front().getFirstNonPHI(),
+      NumInstRemoved += changeToUnreachable(F.front().getFirstNonPHIOrDbg(),
                                             /*PreserveLCSSA=*/false, &DTU);
 
     BasicBlock *NewUnreachableBB = nullptr;

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -2871,10 +2871,16 @@ void InstCombinerImpl::handleUnreachableFrom(
     }
     if (Inst.isEHPad() || Inst.getType()->isTokenTy())
       continue;
+    // RemoveDIs: erase debug-info on this instruction manually.
+    Inst.dropDbgValues();
     eraseInstFromFunction(Inst);
     MadeIRChange = true;
   }
 
+  // RemoveDIs: to match behaviour in dbg.value mode, drop debug-info on
+  // terminator too.
+  BB->getTerminator()->dropDbgValues();
+
   // Handle potentially dead successors.
   for (BasicBlock *Succ : successors(BB))
     addDeadEdge(BB, Succ, Worklist);

llvm/lib/Transforms/Utils/Local.cpp

@@ -2750,6 +2750,7 @@ unsigned llvm::changeToUnreachable(Instruction *I, bool PreserveLCSSA,
       Updates.push_back({DominatorTree::Delete, BB, UniqueSuccessor});
     DTU->applyUpdates(Updates);
   }
+  BB->flushTerminatorDbgValues();
   return NumInstrsRemoved;
 }
 
@@ -2903,9 +2904,9 @@ static bool markAliveBlocks(Function &F,
           // If we found a call to a no-return function, insert an unreachable
           // instruction after it.  Make sure there isn't *already* one there
           // though.
-          if (!isa<UnreachableInst>(CI->getNextNode())) {
+          if (!isa<UnreachableInst>(CI->getNextNonDebugInstruction())) {
             // Don't insert a call to llvm.trap right before the unreachable.
-            changeToUnreachable(CI->getNextNode(), false, DTU);
+            changeToUnreachable(CI->getNextNonDebugInstruction(), false, DTU);
             Changed = true;
           }
           break;

llvm/test/Transforms/InstCombine/assume.ll

@@ -2,6 +2,9 @@
 ; RUN: opt < %s -passes=instcombine -S | FileCheck --check-prefixes=CHECK,DEFAULT %s
 ; RUN: opt < %s -passes=instcombine --enable-knowledge-retention -S | FileCheck --check-prefixes=CHECK,BUNDLES %s
 
+; RUN: opt < %s -passes=instcombine -S --try-experimental-debuginfo-iterators | FileCheck --check-prefixes=CHECK,DEFAULT %s
+; RUN: opt < %s -passes=instcombine --enable-knowledge-retention -S --try-experimental-debuginfo-iterators | FileCheck --check-prefixes=CHECK,BUNDLES %s
+
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 

llvm/test/Transforms/InstCombine/unreachable-dbg-info-modified.ll

@@ -1,4 +1,5 @@
 ; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+; RUN: opt < %s -passes=instcombine -S --try-experimental-debuginfo-iterators | FileCheck %s
 
 ; When removing the llvm.dbg.value intrinsic in the unreachable block
 ; InstCombine would incorrectly return a false Modified status.

llvm/test/Transforms/SCCP/ipsccp-branch-unresolved-undef.ll

@@ -1,5 +1,8 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --function-signature
 ; RUN: opt < %s -S -passes=ipsccp | FileCheck %s
+; RUN: opt < %s -S -passes=ipsccp --try-experimental-debuginfo-iterators | FileCheck %s
+;; Check that @patatino is optimised to "unreachable" given that it branches on
+;; undef. Check too that debug intrinsics have no effect on this.
 
 define void @main() {
 ; CHECK-LABEL: define {{[^@]+}}@main() {
@@ -17,7 +20,39 @@ define internal i1 @patatino(i1 %a) {
 ;
   br i1 %a, label %ontrue, label %onfalse
 ontrue:
+  call void @llvm.dbg.value(metadata i32 0, metadata !6, metadata !DIExpression()), !dbg !11
+  call void @llvm.dbg.value(metadata i32 1, metadata !9, metadata !DIExpression()), !dbg !11
   ret i1 false
 onfalse:
+  call void @llvm.dbg.value(metadata i32 0, metadata !6, metadata !DIExpression()), !dbg !11
+  call void @llvm.dbg.value(metadata i32 1, metadata !9, metadata !DIExpression()), !dbg !11
   ret i1 false
 }
+
+declare void @llvm.dbg.value(metadata, metadata, metadata) nounwind readnone
+
+!llvm.module.flags = !{!21}
+!llvm.dbg.cu = !{!2}
+
+!0 = distinct !DISubprogram(name: "foo", line: 2, isLocal: false, isDefinition: true, virtualIndex: 6, flags: DIFlagPrototyped, isOptimized: false, unit: !2, file: !20, scope: !1, type: !3)
+!1 = !DIFile(filename: "b.c", directory: "/private/tmp")
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, producer: "clang", isOptimized: true, emissionKind: FullDebug, file: !20)
+!3 = !DISubroutineType(types: !4)
+!4 = !{!5}
+!5 = !DIBasicType(tag: DW_TAG_base_type, name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
+!6 = !DILocalVariable(name: "i", line: 2, arg: 1, scope: !0, file: !1, type: !5)
+!7 = !DILocation(line: 2, column: 13, scope: !0)
+!9 = !DILocalVariable(name: "k", line: 3, scope: !10, file: !1, type: !5)
+!10 = distinct !DILexicalBlock(line: 2, column: 16, file: !20, scope: !0)
+!11 = !DILocation(line: 3, column: 12, scope: !10)
+!12 = !DILocation(line: 4, column: 3, scope: !10)
+!13 = !DILocation(line: 5, column: 5, scope: !14)
+!14 = distinct !DILexicalBlock(line: 4, column: 10, file: !20, scope: !10)
+!15 = !DILocation(line: 6, column: 3, scope: !14)
+!16 = !DILocation(line: 7, column: 5, scope: !17)
+!17 = distinct !DILexicalBlock(line: 6, column: 10, file: !20, scope: !10)
+!18 = !DILocation(line: 8, column: 3, scope: !17)
+!19 = !DILocation(line: 9, column: 3, scope: !10)
+!20 = !DIFile(filename: "b.c", directory: "/private/tmp")
+!21 = !{i32 1, !"Debug Info Version", i32 3}
+

llvm/test/Transforms/SimplifyCFG/PR27615-simplify-cond-br.ll

@@ -1,6 +1,9 @@
 ; RUN: opt -S -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -strip-debug < %s | FileCheck %s
 ; RUN: opt -S -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 < %s | FileCheck %s
 
+; RUN: opt -S -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -strip-debug < %s --try-experimental-debuginfo-iterators | FileCheck %s
+; RUN: opt -S -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 < %s --try-experimental-debuginfo-iterators | FileCheck %s
+
 ; Test case for BUG-27615
 ; Test that simplify cond branch produce same result for debug and non-debug builds
 ; CHECK: select i1 %or.cond, i32 -1, i32 5

llvm/test/Transforms/SimplifyCFG/branch-fold-dbg.ll

@@ -1,5 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S < %s | FileCheck %s
+; RUN: opt -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S < %s --try-experimental-debuginfo-iterators | FileCheck %s
 
 %0 = type { ptr, ptr }
 

llvm/test/Transforms/SimplifyCFG/dbginfo.ll

@@ -1,5 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S | FileCheck %s
+; RUN: opt < %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S --try-experimental-debuginfo-iterators | FileCheck %s
 
   %llvm.dbg.anchor.type = type { i32, i32 }
   %llvm.dbg.basictype.type = type { i32, ptr, ptr, ptr, i32, i64, i64, i64, i32, i32 }