[llvm-dev] [RFC] Removing debug locations from ConstantSDNodes

[Reid Kleckner via llvm-dev]

On Tue, Jun 19, 2018 at 5:46 PM Vedant Kumar <vsk at apple.com> wrote:

> Someone (Reid?) mentioned that we could try sinking constants to their
> point of first use as an alternative, and (IIUC) create new nodes with
> distinct DebugLocs for each use of a constant. I don't recall the details
> of that alternative clearly. Based on my (likely incorrect) understanding
> of it, dropping locations from constants outright might be simpler.
>

Our use case was in fastisel, so things are different. I don't think my
solution will help you.

In our case, users were complaining about code like this:
volatile int do_something;
void f() {
  ++do_something;
  foo(1, 2, 3);
  ++do_something;
}

We'd generate locations like:
  .loc line 1
  incl do_something(%rip)
  movl $1, %ecx
  movl $2, %edx
  movl $3, %r8d
  .loc line 2 # line 2 starts here, instead of 3 instructions earlier
  callq foo
  .loc line 3
  incl do_something(%rip)

Our users really wanted the line table entry for line 2 to include the
constant materialization code for some VS debugger feature.

I think if you remove locations from ConstantSDNodes, you might want to add
a late pass that propagates source locations backwards onto location-less
instructions. This would also avoid some special cases when a basic block
starts with an instruction that lacks location information.
See CodeViewDebug::beginInstruction and DwarfDebug::beginInstruction for
what we do today.
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