llvm dev - Apr 2011 - [LLVMdev] Transition C->bitcode->assembly->object looses frame pointers

Hi everyone,

We're doing some compile-time instrumentation according to the following
scheme:

  llvm-gcc -O1 -g $in -emit-llvm -S -o $name.ll
  opt  -O2 -load Instr.so $INSTR_FLAGS $name.ll -S -o $name-inst.ll
  llc  $name-inst.ll -o $name.S
  g++ -c $name.S

However it turns out that the code instrumented this way is missing
frame pointers (e.g. backtrace() and/or libunwind cannot unwind the
stack when necessary).
Moreover, removing the instrumentation doesn't help: a simple
transition to and from bitcode doesn't help:

  llvm-gcc -O1 -g $in -emit-llvm -S -o $name.ll
  llc  $name.ll -o $name.S
  g++ -c $name.S

We also tried to use Clang and to add -fno-omit-frame-pointers and
--disable-fp-elim -- do not work either.

On the other hand, compiling the code in a single pass of
gcc/llvm-gcc/clang does preserve the frame pointers.

Does anyone happen to know where the problem might be? Are there any
other flags we're missing?

Thanks,
Alexander Potapenko

Hi,

The -disable-fp-elim flag must be passed to the part of the compilation process
that actually performs that optimisation pass - when you run clang/llvm-gcc in
one invocation, obviously it will just be that invocation, but in your first
case it would be the llc command. That is where the IR gets lowered, so is where
the FP elimination takes place.

Pass flags are not, as far as I'm aware, propagated in generated bitcode
files - the same flags need to be given to each part of the pipeline.

Hope this helps,

James
> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu]
> On Behalf Of Alexander Potapenko
> Sent: 05 April 2011 13:23
> To: llvmdev
> Subject: [LLVMdev] Transition C->bitcode->assembly->object looses
frame
> pointers
>
> Hi everyone,
>
> We're doing some compile-time instrumentation according to the
> following scheme:
>
>   llvm-gcc -O1 -g $in -emit-llvm -S -o $name.ll
>   opt  -O2 -load Instr.so $INSTR_FLAGS $name.ll -S -o $name-inst.ll
>   llc  $name-inst.ll -o $name.S
>   g++ -c $name.S
>
> However it turns out that the code instrumented this way is missing
> frame pointers (e.g. backtrace() and/or libunwind cannot unwind the
> stack when necessary).
> Moreover, removing the instrumentation doesn't help: a simple
> transition to and from bitcode doesn't help:
>
>   llvm-gcc -O1 -g $in -emit-llvm -S -o $name.ll
>   llc  $name.ll -o $name.S
>   g++ -c $name.S
>
> We also tried to use Clang and to add -fno-omit-frame-pointers and
> --disable-fp-elim -- do not work either.
>
> On the other hand, compiling the code in a single pass of
> gcc/llvm-gcc/clang does preserve the frame pointers.
>
> Does anyone happen to know where the problem might be? Are there any
> other flags we're missing?
>
> Thanks,
> Alexander Potapenko
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

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Hi James,

We've indeed passed the appropriate (and even excessive) flags to the
appropriate pipeline parts, that is:

 llvm-gcc -O1 -fno-omit-frame-pointers -g $in -emit-llvm -S -o $name.ll
 llc --disable-fp-elim $name.ll -o $name.S
 g++ -fno-omit-frame-pointers -c $name.S

, but that didn't work

Alex

On Tue, Apr 5, 2011 at 4:40 PM, James Molloy <James.Molloy at arm.com>
wrote:
> Hi,
>
> The -disable-fp-elim flag must be passed to the part of the compilation
process that actually performs that optimisation pass - when you run
clang/llvm-gcc in one invocation, obviously it will just be that invocation, but
in your first case it would be the llc command. That is where the IR gets
lowered, so is where the FP elimination takes place.
>
> Pass flags are not, as far as I'm aware, propagated in generated
bitcode files - the same flags need to be given to each part of the pipeline.
>
> Hope this helps,
>
> James
>

Hi Alexander,
> We're doing some compile-time instrumentation according to the
following scheme:
>
>    llvm-gcc -O1 -g $in -emit-llvm -S -o $name.ll
>    opt  -O2 -load Instr.so $INSTR_FLAGS $name.ll -S -o $name-inst.ll
>    llc  $name-inst.ll -o $name.S
llc eliminates frame pointers by default, see the -disable-fp-elim and
-disable-non-leaf-fp-elim options.

Ciao, Duncan.