Re: How to debug while using LTO?

[Richard Biener <richard.guenther@gmail.com>]

> Am 24.11.2022 um 17:28 schrieb Stefan Schulze Frielinghaus via Gcc <gcc@gcc.gnu.org>:
> 
> Hi everyone,
> 
> Currently I'm looking into a wrong-code bug and would like to understand
> a certain optimization done by combine during local transformation.
> Without LTO I would simply debug cc1 and step through combine.  However,
> with LTO enabled AFAIK I have to debug lto1 instead.  In order to get
> the lto1 command line of interest according to
> https://gcc.gnu.org/legacy-ml/gcc/2009-11/msg00047.html
> I have to pass -Wl,-debug to gcc in order to get the command for
> collect2 to which itself I have to pass -plugin-opt=-debug in order to
> get the command for lto-wrapper.  According to the aforementioned mail I
> should add option -debug to lto-wrapper, however, it appears to me that
> option -debug was removed.  I gave options -v and -### a chance without
> luck, i.e., those only print the usual environment variables and
> afterwards a list of object files like
> 
> /tmp/ccPEIV35.ltrans0.ltrans.o
> /tmp/ccNmpKfS.debug.temp.o
> /tmp/cceiCIFg.debug.temp.o
> /tmp/ccZ4Qc7E.debug.temp.o
> ...
> 
> but no lto1 command.  Thus, how do you retrieve the lto1 command?
> 
> While desperate I retrieved it manually via strace.  However, the lto1
> command refers to temporary files which have been erased meanwhile.  I
> actually didn't expect that because I added -save-temps to all the
> intermediate commands which is also reflected in the environment
> variable COLLECT_GCC_OPTIONS.  Thus, how do you keep temporary files?

Adding -v -save-temps and then running gdb on the lto1 command works and is what I usually do.

Richard 

> Cheers,
> Stefan