How to debug while using LTO?

[Stefan Schulze Frielinghaus]

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?

Cheers,
Stefan