Problem: I misunderstood the purpose of .debug_frames (objdump
--dwarf=frames).
The purpose of .debug_frames is not to record the size of function
stack frames, but to only tell a debugger how to access the previous
function's stack frame. It just so happens that this _coincidentally_
tells you the stack frame size when compiling with -fomit-frame-pointer.
With -fno-omit-frame-pointer (common on some archs), .debug_frames just
says "hey here's the frame pointer" (DW_CFA_def_cfa_register), which
tells us nothing about the function's actual stack usage.
So unfortunately .debug_frames does not provide enough info on its
own...
---
This commit was an attempt to find the actual stack usage by looking at
the relevant variable info (DW_TAG_variable, etc) in function's dwarf
info, but this approach is also not looking very good...
1. The numbers do not appear correct:
before -fcallgraph-info=su: 2720
after --dwarf=info: 3558
after --dwarf=info --no-shrinkwrap: 3922
(this is with -fno-omit-frame-pointer)
In hindsight, this approach is fundamentally flawed. While the
variable tags does give us a lower bound on stack usage, it doesn't
tell us about implicit compiler variables and various stack push/pops
as a part of expression evaluation.
As far as I can tell there's simply not enough info in dwarf info to
find an accurate upper bound on stack usage.
2. This approach is quite a bit more complicated, since we need:
1. Dwarf info (--dwarf=info) to find variable tags.
2. Location info (--dwarf=loc) to map var allocations to address
ranges.
3. Range info (--dwarf=Ranges) to map lexical blocks to address
ranges when var allocation is implicit (not implemented).
4. And we still need frame info (--dwarf=frames)! since var
allocations are frame-relative.
3. Also dwarf info is not guaranteed to contain the whole callgraph.
It seems callgraph info is actually _omitted_ with -O0??
I guess this is because the callgraph info is a side-effect of some
compiler pass? This seems a bit backwards.
Dwarf does have a flag (DW_AT_call_all_calls) to indicate when
callgraph info is complete, but it doesn't seem to be set reliably?
Even with optimizations, lfsr_bd_sync, _and only lfsr_bd_sync_, is
missing the DW_AT_call_all_calls flag. I have no idea why. The flag
is still present in lfsr_bd_erase, lfsr_bd_read, and other functions
with function pointers...
So I think this will probably be reverted.
Christopher Haster
0e658b8246