[Bug c/63625] New: -flto breaks optimization during link phase on arm-none-eabi without -O on linker (!) command line

[anpaza at mail dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63625

            Bug ID: 63625
           Summary: -flto breaks optimization during link phase on
                    arm-none-eabi without -O on linker (!) command line
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: anpaza at mail dot ru

Created attachment 33790
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33790&action=edit
A test source file & build shell script demonstrating the bug

I use arm-none-eabi-gcc (Fedora 2013.11.24-2.fc19) 4.8.1.

When compiling and linking "as usual" with the -flto switch the resulting
executable is not optimized.

It looks like the code gets recompiled again somehow if -flto was specified on
linker command line (I'm not sure how exactly -flto works but I suppose it
relinks the whole code again somewhat like -fwhole-program but from GIMPLE
opcodes).

Since it recompiles everything again, the optimization options have effect on
the result. If you don't specify -O on linker command line (which is useless in
the traditional usage mode), you'll get an absolutely unoptimized executable.

I think the optimization options should be either stored into the object files,
or maybe at least WRITE A BIG RED WARNING in the manual about it. I'm doing
embedded code and was shocked of the "quality" of the code... then I objdump'ed
the object file and have seen a totally different code - clean and optimized. I
almost pulled all my hair off till I realized it was the -flto switch that I
monkey-copied from someone else's makefile and after reading the description in
the manual I decided it "is a good thing".

The attached scripts demonstrate the bug. Comment out the -flto LDFLAGS switch
and the .s output gets optimized, uncomment and it gets unoptimized.