Better merging of -fPIC/pic/PIE/pie in lto-wrapper

[Jan Hubicka]

> On Sat, Jul 8, 2017 at 1:03 PM, Jan Hubicka <hubicka@ucw.cz> wrote:
> > Hi,
> > PR lto/80838 is about lto+profiledbootstrapped compiler being slower than
> > profiledboostrapped compiler.
> >
> > This is caused by a fact that some of object files linked into cc1plus binary
> > are built with -fPIC and lto-wrapper then decides to make whole binary PIC that
> > is very slow.  While we probably ought to avoid linking PIC code into static
> > binary but I saw similar issue with firefox and other programs.
> >
> > I do not think we want to support mixed PIC/non-PIC symbols internally, because
> > it would need quite some work and I do not see any reasonable use cases.
> >
> > This patch makes merging more realistic/agressive.  Linking -fPIC and non-PIC
> > code together results in non-PIC binary and thus the corresponding flags are
> > dropped when mismatches occurs.
> >
> > It would be nice to warn about it, but I do not know how to make warning
> > meaningful on targets that are PIC by default.
> >
> > Bootstrapped/regtested x86_64-linux, plan to commit it tomorrow after
> > lto-boottrapping if there are no complains.
> 
> Hum.  I wonder if/why we can't ask the linker about the output binary kind?
Hmm, you are right. I forgot I implemented also that :).
I bleieve we need both - disable PIC when linker tell us we can and merge option
so we make sane decisions between pic/PIC, pie/PIE. And of course we have the
wonderful non-plugin path.

I bleieve we got code quality regression here because I forgot about clearing flag_shlib
for non-dynamic builds:
Index: lto-lang.c
===================================================================
--- lto-lang.c  (revision 250245)
+++ lto-lang.c  (working copy)
@@ -840,11 +840,13 @@
          flag_pie is 2.  */
       flag_pie = MAX (flag_pie, flag_pic);
       flag_pic = flag_pie;
+      flag_shlib = 0;
       break;
 
     case LTO_LINKER_OUTPUT_EXEC: /* Normal executable */
       flag_pic = 0;
       flag_pie = 0;
+      flag_shlib = 0;
       break;
 
     case LTO_LINKER_OUTPUT_UNKNOWN:

I will test this and verify that there is no code quality difference without the option merging
then.

Honza

> 
> Richard.