Re: curious check in dwarf2_gen_line_info()

[Geoff Keating <geoffk@geoffk.org>]

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On 29/12/2004, at 2:05 AM, David Mosberger wrote:

> [Resend^2: I mistakenly typed .com instead of .org for his domain... 
> ;-( ]
>
> In dwarf2dbg.c:dwarf2_gen_line_info(), one finds this curious code and
> comment:
>
>   /* Don't emit sequences of line symbols for the same line when the
>      symbols apply to assembler code.  It is necessary to emit
>      duplicate line symbols when a compiler asks for them, because GDB
>      uses them to determine the end of the prologue.  */
>   if (debug_type == DEBUG_DWARF2
>         && line == loc->line && filenum == loc->filenum)
>     return;
>
> The "debug_type == DEBUG_DWARF2" checks has the effect that for
> compiler-generated files, the line info is replicated for each
> instruction, even when that info doesn't change at all.
>
> The change seems to have been introduced back in 2002:
>
>   2002-04-17  Geoffrey Keating  <geoffk@redhat.com>
>         * dwarf2dbg.c (dwarf2_gen_line_info): Do emit duplicate line
>         numbers, gdb relies on them to detect the start of the 
> prologue.
>
> but there is no hint on which platform needed this workaround.

My understanding is that it was needed on all platforms.  It's used 
when the user writes

break foo

in GDB, to break after foo's prologue, rather than on its first 
instruction.  As I wrote in 2002,

> GDB uses duplicate line numbers to detect the end of the prologue.
> GCC would carefully emit them... and gas was stripping them out.
>
> I don't believe this patch will significantly affect the size of the
> debug info for assembler source, because having multiple instructions
> on a line is rare.
>
> I tested this by running the GAS and GDB testsuites on
> powerpc-eabisim.

[the patch was later revised to not apply to assembler source.]

Maybe the GDB people have implemented some other technique for this, in 
which case it could go away.  Running the GDB testsuite should show the 
problem.  (Actually, IMO, the whole thing is wrong; GDB really should 
be breaking on the first instruction.  The problem with that is that 
GCC+GDB isn't smart enough to permit the user to print the contents of 
incoming arguments at a breakpoint on the first instruction of a 
routine, it needs the prologue to be executed.  It would be very cool 
to fix that.)

This is not a change that should be first made in the assembler.  
Instead of having the compiler emit duplicates and have the assembler 
remove them, the compiler should be changed to not emit duplicates in 
the first place if they are no longer needed.

The overhead of the duplicates should not be very significant (compared 
to the information that is required), because the compiler should 
already be suppressing duplicate line information except where it is 
needed.

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