[PATCH] D64971: [SafeStack] Don't re-insert derefs for allocas in debug info

[Petr Hosek via Phabricator via llvm-commits]

phosek created this revision.
phosek added a reviewer: eugenis.
Herald added subscribers: llvm-commits, hiraditya, aprantl.
Herald added a project: LLVM.

While debugging code that uses SafeStack, we've noticed that LLVM
produces an invalid DWARF. Concretely, in the following example:

  int main(int argc, char* argv[]) {
    std::string value = "";
    printf("%s\n", value.c_str());
    return 0;
  }

DWARF would describe the value variable as being located at:

  DW_OP_breg14 R14+0, DW_OP_deref, DW_OP_constu 0x20, DW_OP_minus

The assembly to get this variable is:

  leaq    -32(%r14), %rbx

The order of operations in the DWARF symbols is incorrect in this case.
Specifically, the deref is incorrect; this appears to be incorrectly
re-inserted in repalceOneDbgValueForAlloca.

With this change which avoids re-inserting deref, LLVM produces correct
DWARF:

  DW_OP_breg14 R14-32


Repository:
  rL LLVM

https://reviews.llvm.org/D64971

Files:
  llvm/lib/Transforms/Utils/Local.cpp
  llvm/test/Transforms/SafeStack/X86/debug-loc2.ll


Index: llvm/test/Transforms/SafeStack/X86/debug-loc2.ll
===================================================================
--- llvm/test/Transforms/SafeStack/X86/debug-loc2.ll
+++ llvm/test/Transforms/SafeStack/X86/debug-loc2.ll
@@ -25,7 +25,7 @@
   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_deref, DW_OP_constu, 4, DW_OP_minus))
+; CHECK: call void @llvm.dbg.value(metadata i8* %[[USP]], metadata ![[X1:.*]], metadata !DIExpression(DW_OP_constu, 4, DW_OP_minus))
   tail call void @llvm.dbg.value(metadata i32* %x1, metadata !10, metadata !15), !dbg !16
   call void @capture(i32* nonnull %x1), !dbg !17
 
@@ -33,7 +33,7 @@
 ; CHECK: call void @llvm.random.metadata.use(metadata ![[EMPTY]])
   call void @llvm.random.metadata.use(metadata i32* %x2)
 
-; CHECK: call void @llvm.dbg.value(metadata i8* %[[USP]], metadata ![[X2:.*]], metadata !DIExpression(DW_OP_deref, DW_OP_constu, 8, DW_OP_minus))
+; CHECK: call void @llvm.dbg.value(metadata i8* %[[USP]], metadata ![[X2:.*]], metadata !DIExpression(DW_OP_constu, 8, DW_OP_minus))
   call void @llvm.dbg.value(metadata i32* %x2, metadata !12, metadata !15), !dbg !18
   call void @capture(i32* nonnull %x2), !dbg !19
   ret void, !dbg !20
Index: llvm/lib/Transforms/Utils/Local.cpp
===================================================================
--- llvm/lib/Transforms/Utils/Local.cpp
+++ llvm/lib/Transforms/Utils/Local.cpp
@@ -1591,11 +1591,10 @@
       DIExpr->getElement(0) != dwarf::DW_OP_deref)
     return;
 
-  // Insert the offset immediately after the first deref.
+  // Replace the dereference with an offset.
   // We could just change the offset argument of dbg.value, but it's unsigned...
   if (Offset) {
-    SmallVector<uint64_t, 4> Ops;
-    Ops.push_back(dwarf::DW_OP_deref);
+    SmallVector<uint64_t, 3> Ops;
     DIExpression::appendOffset(Ops, Offset);
     Ops.append(DIExpr->elements_begin() + 1, DIExpr->elements_end());
     DIExpr = Builder.createExpression(Ops);


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