[SafeStack] Insert the deref before remaining elements

[phosek]

Summary

This is a follow up to D64971. While we need to insert the deref after
the offset, it needs to come before the remaining elements in the
original expression since the deref needs to happen before the LLVM
fragment if present.

[vsk]

Mind extending debug-loc2.ll with, say, a '!DIExpression(DW_OP_deref, DW_OP_LLVM_fragment, 0, 32)' example?

llvm/lib/Transforms/Utils/Local.cpp

1601–1602

Can this whole block be replaced with: 'DIExpr = DIExpression::prepend(DIExpr, 0, Offset)'? It should handle fragments correctly and avoid inserting any extra derefs.

[phosek]

Done

[vsk]

Thanks a lot, lgtm.

llvm/lib/Transforms/Utils/Local.cpp

static void replaceOneDbgValueForAlloca(DbgValueInst *DVI, Value *NewAddress,
   // This is an alloca-based llvm.dbg.value. The first thing it should do with
   // the alloca pointer is dereference it. Otherwise we don't know how to handle
   // it and give up.
   if (!DIExpr || DIExpr->getNumElements() < 1 ||
       DIExpr->getElement(0) != dwarf::DW_OP_deref)
     return;
 
   // Insert the offset before the first deref.
   // We could just change the offset argument of dbg.value, but it's unsigned...
-  if (Offset) {
-    SmallVector<uint64_t, 4> Ops;
-    DIExpression::appendOffset(Ops, Offset);
-    Ops.append(DIExpr->elements_begin() + 1, DIExpr->elements_end());
-    Ops.push_back(dwarf::DW_OP_deref);
-    DIExpr = Builder.createExpression(Ops);
-  }
+  if (Offset)
+    DIExpr = DIExpression::prepend(DIExpr, 0, Offset);
 
   Builder.insertDbgValueIntrinsic(NewAddress, DIVar, DIExpr, Loc, DVI);
   DVI->eraseFromParent();
 }
 
 void llvm::replaceDbgValueForAlloca(AllocaInst *AI, Value *NewAllocaAddress,
                                     DIBuilder &Builder, int Offset) {
   if (auto *L = LocalAsMetadata::getIfExists(AI))

llvm/test/Transforms/SafeStack/X86/debug-loc2.ll

 ; CHECK: call void @llvm.dbg.value(metadata ![[EMPTY:.*]], metadata !{{.*}}, metadata !{{.*}})
   tail call void @llvm.dbg.value(metadata i32* %x1, metadata !10, metadata !23), !dbg !16
 
 ; Unhandled dbg.value: expression does not start with OP_DW_deref
 ; CHECK: call void @llvm.dbg.value(metadata ![[EMPTY]], metadata !{{.*}}, metadata !{{.*}})
   tail call void @llvm.dbg.value(metadata i32* %x1, metadata !10, metadata !24), !dbg !16
 
 ; Supported dbg.value: rewritted based on the [[USP]] value.
+; CHECK: call void @llvm.dbg.value(metadata i8* %[[USP]], metadata ![[X1:.*]], metadata !DIExpression(DW_OP_constu, 4, DW_OP_minus, DW_OP_deref, DW_OP_LLVM_fragment, 0, 4))
+  tail call void @llvm.dbg.value(metadata i32* %x1, metadata !10, metadata !25), !dbg !16
+
+; Supported dbg.value: rewritted based on the [[USP]] value.
 ; CHECK: call void @llvm.dbg.value(metadata i8* %[[USP]], metadata ![[X1:.*]], metadata !DIExpression(DW_OP_constu, 4, DW_OP_minus, DW_OP_deref))
   tail call void @llvm.dbg.value(metadata i32* %x1, metadata !10, metadata !15), !dbg !16
   call void @capture(i32* nonnull %x1), !dbg !17
 
 ; An extra non-dbg.value metadata use of %x2. Replaced with an empty metadata.
 ; CHECK: call void @llvm.random.metadata.use(metadata ![[EMPTY]])
   call void @llvm.random.metadata.use(metadata i32* %x2)
 

 !17 = !DILocation(line: 8, column: 3, scope: !6)
 !18 = !DILocation(line: 6, column: 7, scope: !6)
 !19 = !DILocation(line: 9, column: 3, scope: !6)
 !20 = !DILocation(line: 10, column: 1, scope: !6)
 !21 = !DILocation(line: 10, column: 1, scope: !22)
 !22 = !DILexicalBlockFile(scope: !6, file: !1, discriminator: 1)
 !23 = !DIExpression()
 !24 = !DIExpression(DW_OP_constu, 42, DW_OP_minus)
+!25 = !DIExpression(DW_OP_deref, DW_OP_LLVM_fragment, 0, 4)