[PATCH] D73526: [SafeStack][DebugInfo] Insert DW_OP_deref in correct location

Tue Feb 4 10:49:33 PST 2020

bjope added a comment.

In D73526#1857223 <https://reviews.llvm.org/D73526#1857223>, @bjope wrote:

> This broke some of our downstream tests using vla:s. Those are compiled from C and executed in gdb. What happens is that DW_OP_deref disappears and then the debugger prints the wrong value.

Oh, maybe I should have mentioned that the regression I found was related to -O1 or higher.

If I run

> llc -O1 -mtriple=x86_64-unknown-linux-gnu -o -  test/DebugInfo/X86/safestack-deref.ll

then I guess I see the same regression:

Before this patch we get

  .Ltmp0:
          #DEBUG_VALUE: func:value <- [DW_OP_constu 8, DW_OP_minus, DW_OP_deref] $rbx

After this patch we get

  .Ltmp0:
          #DEBUG_VALUE: func:value <- [DW_OP_constu 8, DW_OP_minus] $rbx

So the deref has not only moved to the other side of the subtraction, it has disappeared completely.

Maybe there is some difference between FastISel and "normal" ISel here?

Btw, I agree that doing append (like the old code) also seems wrong in case the Expr isn't empty. What was the problem with doing prepend instead?

Repository:
  rG LLVM Github Monorepo

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