[Instruction] Add dropLocation and updateLocationAfterHoist helpers

Introduce a helper which can be used to update the debug location of an
Instruction after the instruction is hoisted. This can be used to safely
drop a source location as recommended by the docs.

For more context, see the discussion in https://reviews.llvm.org/D60913.

Differential Revision: https://reviews.llvm.org/D85670

Author: vedantk

llvm/include/llvm/IR/Instruction.h

@@ -492,6 +492,20 @@ class Instruction : public User,
   /// merged DebugLoc.
   void applyMergedLocation(const DILocation *LocA, const DILocation *LocB);
 
+  /// Updates the debug location given that the instruction has been hoisted
+  /// from a block to a predecessor of that block.
+  /// Note: it is undefined behavior to call this on an instruction not
+  /// currently inserted into a function.
+  void updateLocationAfterHoist();
+
+  /// Drop the instruction's debug location. This does not guarantee removal
+  /// of the !dbg source location attachment, as it must set a line 0 location
+  /// with scope information attached on call instructions. To guarantee
+  /// removal of the !dbg attachment, use the \ref setDebugLoc() API.
+  /// Note: it is undefined behavior to call this on an instruction not
+  /// currently inserted into a function.
+  void dropLocation();
+
 private:
   /// Return true if we have an entry in the on-the-side metadata hash.
   bool hasMetadataHashEntry() const {

llvm/lib/IR/DebugInfo.cpp

@@ -696,6 +696,38 @@ void Instruction::applyMergedLocation(const DILocation *LocA,
   setDebugLoc(DILocation::getMergedLocation(LocA, LocB));
 }
 
+void Instruction::updateLocationAfterHoist() { dropLocation(); }
+
+void Instruction::dropLocation() {
+  const DebugLoc &DL = getDebugLoc();
+  if (!DL)
+    return;
+
+  // If this isn't a call, drop the location to allow a location from a
+  // preceding instruction to propagate.
+  if (!isa<CallBase>(this)) {
+    setDebugLoc(DebugLoc());
+    return;
+  }
+
+  // Set a line 0 location for calls to preserve scope information in case
+  // inlining occurs.
+  const DISubprogram *SP = getFunction()->getSubprogram();
+  if (SP)
+    // If a function scope is available, set it on the line 0 location. When
+    // hoisting a call to a predecessor block, using the function scope avoids
+    // making it look like the callee was reached earlier than it should be.
+    setDebugLoc(DebugLoc::get(0, 0, SP));
+  else
+    // The parent function has no scope. Go ahead and drop the location. If
+    // the parent function is inlined, and the callee has a subprogram, the
+    // inliner will attach a location to the call.
+    //
+    // One alternative is to set a line 0 location with the existing scope and
+    // inlinedAt info. The location might be sensitive to when inlining occurs.
+    setDebugLoc(DebugLoc());
+}
+
 //===----------------------------------------------------------------------===//
 // LLVM C API implementations.
 //===----------------------------------------------------------------------===//

llvm/lib/Transforms/Scalar/GVN.cpp

@@ -48,7 +48,6 @@
 #include "llvm/IR/Constant.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
-#include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/IR/DebugLoc.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Function.h"
@@ -1323,8 +1322,7 @@ bool GVN::PerformLoadPRE(LoadInst *LI, AvailValInBlkVect &ValuesPerBlock,
     // Instructions that have been inserted in predecessor(s) to materialize
     // the load address do not retain their original debug locations. Doing
     // so could lead to confusing (but correct) source attributions.
-    if (const DebugLoc &DL = I->getDebugLoc())
-      I->setDebugLoc(DebugLoc::get(0, 0, DL.getScope(), DL.getInlinedAt()));
+    I->updateLocationAfterHoist();
 
     // FIXME: We really _ought_ to insert these value numbers into their
     // parent's availability map.  However, in doing so, we risk getting into

llvm/lib/Transforms/Scalar/LICM.cpp

@@ -1736,10 +1736,7 @@ static void hoist(Instruction &I, const DominatorTree *DT, const Loop *CurLoop,
     // Move the new node to the destination block, before its terminator.
     moveInstructionBefore(I, *Dest->getTerminator(), *SafetyInfo, MSSAU, SE);
 
-  // Apply line 0 debug locations when we are moving instructions to different
-  // basic blocks because we want to avoid jumpy line tables.
-  if (const DebugLoc &DL = I.getDebugLoc())
-    I.setDebugLoc(DebugLoc::get(0, 0, DL.getScope(), DL.getInlinedAt()));
+  I.updateLocationAfterHoist();
 
   if (isa<LoadInst>(I))
     ++NumMovedLoads;

llvm/test/DebugInfo/Generic/licm-hoist-debug-loc.ll

@@ -18,9 +18,8 @@
 ; We make sure that the instruction that is hoisted into the preheader
 ; does not have a debug location.
 ; CHECK: for.body.lr.ph:
-; CHECK: getelementptr{{.*}}%p.addr, i64 4{{.*}} !dbg [[zero:![0-9]+]]
+; CHECK: getelementptr{{.*}}%p.addr, i64 4{{$}}
 ; CHECK: for.body:
-; CHECK: [[zero]] = !DILocation(line: 0
 ;
 ; ModuleID = 't.ll'
 source_filename = "test.c"

llvm/test/Transforms/GVN/PRE/phi-translate.ll

@@ -4,8 +4,8 @@ target datalayout = "e-p:64:64:64"
 
 ; CHECK-LABEL: @foo(
 ; CHECK: entry.end_crit_edge:
-; CHECK: %[[INDEX:[a-z0-9.]+]] = sext i32 %x to i64{{.*}} !dbg [[ZERO_LOC:![0-9]+]]
-; CHECK: %[[ADDRESS:[a-z0-9.]+]] = getelementptr [100 x i32], [100 x i32]* @G, i64 0, i64 %[[INDEX]]{{.*}} !dbg [[ZERO_LOC]]
+; CHECK: %[[INDEX:[a-z0-9.]+]] = sext i32 %x to i64{{$}}
+; CHECK: %[[ADDRESS:[a-z0-9.]+]] = getelementptr [100 x i32], [100 x i32]* @G, i64 0, i64 %[[INDEX]]{{$}}
 ; CHECK:   %n.pre = load i32, i32* %[[ADDRESS]], align 4, !dbg [[N_LOC:![0-9]+]]
 ; CHECK: br label %end
 ; CHECK: then:
@@ -14,8 +14,7 @@ target datalayout = "e-p:64:64:64"
 ; CHECK:   %n = phi i32 [ %n.pre, %entry.end_crit_edge ], [ %z, %then ], !dbg [[N_LOC]]
 ; CHECK:   ret i32 %n
 
-; CHECK-DAG: [[N_LOC]] = !DILocation(line: 47, column: 1, scope: !{{.*}})
-; CHECK-DAG: [[ZERO_LOC]] = !DILocation(line: 0
+; CHECK: [[N_LOC]] = !DILocation(line: 47, column: 1, scope: !{{.*}})
 
 @G = external global [100 x i32]
 define i32 @foo(i32 %x, i32 %z) !dbg !6 {

llvm/test/Transforms/LICM/hoisting-preheader-debugloc.ll

@@ -1,6 +1,6 @@
 ; RUN: opt -S -licm < %s | FileCheck %s
 
-; CHECK: %arrayidx4.promoted = load i32, i32* %arrayidx4, align 1, !tbaa !59
+; CHECK: %arrayidx4.promoted = load i32, i32* %arrayidx4, align 1, !tbaa !{{[0-9]+$}}
 
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"

llvm/unittests/IR/InstructionsTest.cpp

@@ -13,6 +13,7 @@
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
+#include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/IRBuilder.h"
@@ -1304,5 +1305,76 @@ TEST(InstructionsTest, UnaryOperator) {
   I->deleteValue();
 }
 
+TEST(InstructionsTest, DropLocation) {
+  LLVMContext C;
+  std::unique_ptr<Module> M = parseIR(C,
+                                      R"(
+      declare void @callee()
+
+      define void @no_parent_scope() {
+        call void @callee()           ; I1: Call with no location.
+        call void @callee(), !dbg !11 ; I2: Call with location.
+        ret void, !dbg !11            ; I3: Non-call with location.
+      }
+
+      define void @with_parent_scope() !dbg !8 {
+        call void @callee()           ; I1: Call with no location.
+        call void @callee(), !dbg !11 ; I2: Call with location.
+        ret void, !dbg !11            ; I3: Non-call with location.
+      }
+
+      !llvm.dbg.cu = !{!0}
+      !llvm.module.flags = !{!3, !4}
+      !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+      !1 = !DIFile(filename: "t2.c", directory: "foo")
+      !2 = !{}
+      !3 = !{i32 2, !"Dwarf Version", i32 4}
+      !4 = !{i32 2, !"Debug Info Version", i32 3}
+      !8 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 1, type: !9, isLocal: false, isDefinition: true, scopeLine: 1, isOptimized: false, unit: !0, retainedNodes: !2)
+      !9 = !DISubroutineType(types: !10)
+      !10 = !{null}
+      !11 = !DILocation(line: 2, column: 7, scope: !8, inlinedAt: !12)
+      !12 = !DILocation(line: 3, column: 8, scope: !8)
+  )");
+  ASSERT_TRUE(M);
+
+  {
+    Function *NoParentScopeF =
+        cast<Function>(M->getNamedValue("no_parent_scope"));
+    BasicBlock &BB = NoParentScopeF->front();
+
+    auto *I1 = BB.getFirstNonPHI();
+    auto *I2 = I1->getNextNode();
+    auto *I3 = BB.getTerminator();
+
+    EXPECT_EQ(I1->getDebugLoc(), DebugLoc());
+    I1->dropLocation();
+    EXPECT_EQ(I1->getDebugLoc(), DebugLoc());
+
+    EXPECT_EQ(I2->getDebugLoc().getLine(), 2U);
+    I2->dropLocation();
+    EXPECT_EQ(I1->getDebugLoc(), DebugLoc());
+
+    EXPECT_EQ(I3->getDebugLoc().getLine(), 2U);
+    I3->dropLocation();
+    EXPECT_EQ(I3->getDebugLoc(), DebugLoc());
+  }
+
+  {
+    Function *WithParentScopeF =
+        cast<Function>(M->getNamedValue("with_parent_scope"));
+    BasicBlock &BB = WithParentScopeF->front();
+
+    auto *I2 = BB.getFirstNonPHI()->getNextNode();
+
+    MDNode *Scope = cast<MDNode>(WithParentScopeF->getSubprogram());
+    EXPECT_EQ(I2->getDebugLoc().getLine(), 2U);
+    I2->dropLocation();
+    EXPECT_EQ(I2->getDebugLoc().getLine(), 0U);
+    EXPECT_EQ(I2->getDebugLoc().getScope(), Scope);
+    EXPECT_EQ(I2->getDebugLoc().getInlinedAt(), nullptr);
+  }
+}
+
 } // end anonymous namespace
 } // end namespace llvm