[Assignment Tracking] Fix assertion in AssignmentTrackingPass::runOnFunction

[Orlando]

Summary

The assertion exists to ensure all variables passed into trackAssignments end up with dbg.assigns associated with their backing allocas. The assertion compared the passed-in and tracked variables using DebugVariable, which includes the fragment as part of the variable identity.

It is possible for the backing alloca to be smaller than a variable (see test case). In this case the input variable (Var X, no fragment, no InlinedAt) isn't equal to the dbg.assign variable (Var X, some fragment, no InlinedAt). To cover this case the assertion now ignores fragments through the use of DebugVariableAggregate.

[jmorse]

LGTM, with nit

llvm/lib/IR/DebugInfo.cpp

2035

It's worth dropping a comment here indicating why you've used DebugVariableAggregate instead of DebugVariable, to let the reader work out that the fragment can be changed by AT.

llvm/lib/IR/DebugInfo.cpp

bool AssignmentTrackingPass::runOnFunction(Function &F) {
 
   // Delete dbg.declares for variables now tracked with assignment tracking.
   for (auto &P : DbgDeclares) {
     const AllocaInst *Alloca = P.first;
     auto Markers = at::getAssignmentMarkers(Alloca);
     (void)Markers;
     for (DbgDeclareInst *DDI : P.second) {
       // Assert that the alloca that DDI uses is now linked to a dbg.assign
-      // describing the same variable (i.e. check that this dbg.declare
-      // has been replaced by a dbg.assign).
+      // describing the same variable (i.e. check that this dbg.declare has
+      // been replaced by a dbg.assign). Use DebugVariableAggregate to Discard
+      // the fragment part because trackAssignments may alter the
+      // fragment. e.g. if the alloca is smaller than the variable, then
+      // trackAssignments will create an alloca-sized fragment for the
+      // dbg.assign.
       assert(llvm::any_of(Markers, [DDI](DbgAssignIntrinsic *DAI) {
-        return DebugVariable(DAI) == DebugVariable(DDI);
+        return DebugVariableAggregate(DAI) == DebugVariableAggregate(DDI);
       }));
       // Delete DDI because the variable location is now tracked using
       // assignment tracking.
       DDI->eraseFromParent();
       Changed = true;
     }
   }
   return Changed;

llvm/test/DebugInfo/Generic/assignment-tracking/declare-to-assign/long-double-x87.ll

+; RUN: opt %s -S -passes=declare-to-assign -o - | FileCheck %s
+
+;; Generated from this C++:
+;; long double get();
+;; void fun() {
+;;   long double f;
+;; }
+
+;; Check a variable that is larger (128 bits according to !15) than its backing
+;; alloca (80 bits) can be represented with assignment tracking. Create a
+;; fragment for the dbg.assign for bits 0-80.
+
+; CHECK: llvm.dbg.assign(metadata i1 undef, metadata ![[#]], metadata !DIExpression(DW_OP_LLVM_fragment, 0, 80), metadata ![[#]], metadata ptr %f, metadata !DIExpression())
+
+define dso_local void @_Z3funv() #0 !dbg !10 {
+entry:
+  %f = alloca x86_fp80, align 16
+  call void @llvm.dbg.declare(metadata ptr %f, metadata !14, metadata !DIExpression()), !dbg !16
+  ret void
+}
+
+declare void @llvm.dbg.declare(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3}
+!llvm.ident = !{!9}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 17.0.0", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.cpp", directory: "/")
+!2 = !{i32 7, !"Dwarf Version", i32 5}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!9 = !{!"clang version 17.0.0"}
+!10 = distinct !DISubprogram(name: "fun", linkageName: "_Z3funv", scope: !1, file: !1, line: 2, type: !11, scopeLine: 2, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !13)
+!11 = !DISubroutineType(types: !12)
+!12 = !{null}
+!13 = !{}
+!14 = !DILocalVariable(name: "f", scope: !10, file: !1, line: 3, type: !15)
+!15 = !DIBasicType(name: "long double", size: 128, encoding: DW_ATE_float)
+!16 = !DILocation(line: 3, column: 15, scope: !10)