[cfe-dev] RFC: Remove uninteresting debug locations at -O0

David Blaikie via cfe-dev cfe-dev at lists.llvm.org
Tue Apr 28 14:20:47 PDT 2020

On Tue, Apr 28, 2020 at 1:57 PM Adrian Prantl <aprantl at apple.com> wrote:

> Getting source location information right is tricky and all about finding
> a balance.
> Recently, I was wondering why stepping through this contrived example
>    1       struct Foo {
>    2         Foo *getFoo() { return this; }
>    3       };
>    4
>    5       int main(int argc, char **argv) {
>    6         Foo *foo = new Foo();
>    7         foo->getFoo()->getFoo();
>    8         return 0;
>    9       }
> LLDB was showing the column marker as
>    7         foo->getFoo()->getFoo();
>              ^^^
> focussing on foo instead of at the method call getFoo() that I was
> expecting.
> In LLVM IR, this code looks like
>   %1 = load %struct.Foo*, %struct.Foo** %foo, align 8, !dbg !30
>   %call1 = call %struct.Foo* @_ZN3Foo6getFooEv(%struct.Foo* %1), !dbg !31
>   %call2 = call %struct.Foo* @_ZN3Foo6getFooEv(%struct.Foo* %call1), !dbg
> !32
> or, in x86_64 assembler:
>   .loc 1 7 3 is_stmt 1 ## column_info.cpp:7:3
>   movq -24(%rbp), %rdi
>   .loc 1 7 8 is_stmt 0 ## column_info.cpp:7:8
>   callq __ZN3Foo6getFooEv
>   .loc 1 7 18 ## column_info.cpp:7:18
> The spurious (7:3) location is attached to an instruction that is the load
> of the variable from the stack slot, fused with moving that value into the
> register the ABI defines for $arg0.
> I’m postulating that the source location of the LLVM IR load is
> uninteresting and perhaps even harmful. It is uninteresting, because at
> -O0, the location does not refer to explicit code that the user wrote and
> thus causes unintuitive stepping, and with optimizations enabled, there is
> a high likelihood that the entire instruction is going to be eliminated
> because of mem2reg, so the effect on profiling should be minimal. Since the
> load is from an alloca, it also cannot crash under normal operation. The
> location is harmful, because loads (at least on a CISC instruction set) are
> often fused with other instructions and having conflicting locations will
> cause both locations to be dropped when merged.
> Based on all this I would most like to assign a form of “weak” source
> location to loads from allocas generated by the Clang frontend, that looses
> against any other source location when merged. The closest thing we have to
> this in LLVM IR is attaching no debug location. An instruction without a
> debug location either belongs to the function prologue, or will inherit
> whatever debug location the instruction before it has. In this particular
> case I think that no debug location is preferable over line 0 (which is how
> we usually denote compiler-generated code) because we don’t want the load
> instruction’s source location to erase any source location it may get
> merged with. One thing I need to check is what happens when an instruction
> without a location is the first instruction in a basic block. We may need
> to make an exception for that case.
> To summarize, I’m proposing to delete all debug locations from
> instructions generated in the Clang frontend for loads from allocas that
> are holding source variables to improve the debug experience at -O0. This
> will have little effect on optimized code.
> Let me know what you think!

In general, pretty apprehensive - in terms of practical effects that come
to mind, seems like this might break sanitizers that want to describe
specific loads/stores (even for local memory allocations - wonder how it
looks for something like MSan). There is some work on value based
profiling, I think, too? But I don't know much about it & whether this
would impact that.

Could you show a more motivating example? I assume when you're actually
inside the getFoo call, going up one frame would give you a line/column
that points to the specific getFoo function call, yes? So this is only
about stepping, but that step doesn't look particularly problematic to me -
perhaps it gets worse/confusing/misleading in other places?
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