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

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
>

Below is an example of using backtrace() obtained from man backtrace
and the test results:

 $ gcc bt.c -o bt-gcc
 $ ./bt-gcc 4
backtrace() returned 4 addresses
./bt-gcc() [0x4007e3]
./bt-gcc() [0x400874]
./bt-gcc() [0x40089b]
./bt-gcc() [0x400894]

 $ llvm-gcc bt.c -o bt-llvm-gcc
 $ ./bt-llvm-gcc 4
backtrace() returned 4 addresses
./bt-llvm-gcc() [0x40074b]
./bt-llvm-gcc() [0x400809]
./bt-llvm-gcc() [0x400837]
./bt-llvm-gcc() [0x400830]

 $ llvm-gcc -fno-omit-frame-pointer -g bt.c -emit-llvm -S -o bt.ll
 $ llc --disable-fp-elim bt.ll -o bt.S
 $ gcc -fno-omit-frame-pointer bt.S -o bt-llc
 $ ./bt-llc 4
backtrace() returned 1 addresses
./bt-llc() [0x4007f0]

 $ cat bt.c
       #include <execinfo.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

       void
       myfunc3(void)
       {
           int j, nptrs;
       #define SIZE 100
           void *buffer[100];
           char **strings;

           nptrs = backtrace(buffer, SIZE);
           printf("backtrace() returned %d addresses\n", nptrs);

           /* The call backtrace_symbols_fd(buffer, nptrs, STDOUT_FILENO)
              would produce similar output to the following: */

           strings = backtrace_symbols(buffer, nptrs);
           if (strings == NULL) {
               perror("backtrace_symbols");
               exit(EXIT_FAILURE);
           }

           for (j = 0; j < nptrs; j++)
               printf("%s\n", strings[j]);

           free(strings);
       }

       static void   /* "static" means don't export the symbol...
*/
       myfunc2(void)
       {
           myfunc3();
       }

       void
       myfunc(int ncalls)
       {
           if (ncalls > 1)
               myfunc(ncalls - 1);
           else
               myfunc2();
       }

       int
       main(int argc, char *argv[])
       {
           if (argc != 2) {
               fprintf(stderr, "%s num-calls\n", argv[0]);
               exit(EXIT_FAILURE);
           }

           myfunc(atoi(argv[1]));
           exit(EXIT_SUCCESS);
       }


On Tue, Apr 5, 2011 at 4:49 PM, Alexander Potapenko
<ramosian.glider at gmail.com> wrote:
> 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'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
it should work, so at this point I suggest you provide a concrete example
in which frame pointers are nonetheless eliminated.

Ciao, Duncan.

Hi Alexander, while I can reproduce the problem with your example,
inspection of the assembler shows that the frame pointer is being
correctly set up in each function.  So I don't think the problem
is due to frame pointer elimination.

Ciao, Duncan.