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[SafeStack] Insert the deref after the offset 

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 inserts the deref after the offset instead of
before it, LLVM produces correct DWARF:

  DW_OP_breg14 R14-32

Differential Revision: https://reviews.llvm.org/D64971

llvm-svn: 366726
This commit is contained in:
Petr Hosek 2019-07-22 18:52:42 +00:00
parent ef5cfc2dae
commit f6cd6ffbc9
2 changed files with 4 additions and 4 deletions
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4
llvm/lib/Transforms/Utils/Local.cpp
View File

@ -1597,13 +1597,13 @@ static void replaceOneDbgValueForAlloca(DbgValueInst *DVI, Value *NewAddress,
DIExpr->getElement(0) != dwarf::DW_OP_deref)
return;
// Insert the offset immediately after the first deref.
// 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;
Ops.push_back(dwarf::DW_OP_deref);
DIExpression::appendOffset(Ops, Offset);
Ops.append(DIExpr->elements_begin() + 1, DIExpr->elements_end());
Ops.push_back(dwarf::DW_OP_deref);
DIExpr = Builder.createExpression(Ops);
}

4
llvm/test/Transforms/SafeStack/X86/debug-loc2.ll
View File

@ -25,7 +25,7 @@ entry:
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, 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
@ -33,7 +33,7 @@ entry:
; 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, DW_OP_deref))
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