[RS6000] Correct save_reg_p

[Alan Modra]

Fixes lack of r30 save/restore on

// -m32 -fpic -ftls-model=initial-exec
__thread char* p;
char** f1 (void) { return &p; }

and

// -m32 -fpic -msecure-plt
extern int foo (int);
int f1 (int x) { return foo (x); }

These are both caused by save_reg_p returning false when the pic
offset table reg (r30 for ABI_V4) was used, due to the logic not
exactly matching that in rs6000_emit_prologue to set up r30.  I've
simplified the ABI_V4 logic down to df_regs_ever_live_p since that is
obviously correct for all ABIs.

I also noticed that save_reg_p isn't following the comment regarding
calls_eh_return (since svn 267049, git 0edf78b1b2a0), and the comment
needs tweaking too.  For why the revised comment is correct, grep for
saves_all_registers in lra.c, and yes, we do want to save the pic
offset table reg for eh_return.

Bootstrap and regression test on powerpc64-linux biarch in progress.
OK assuming no regressions?  Segher this patch supercedes
https://gcc.gnu.org/ml/gcc-patches/2019-02/msg00307.html as there's no
need to set crtl->uses_pic_offset_table for those cases now with the
df_regs_ever_live_p test in save_reg_p.

        PR target/88343
        * config/rs6000/rs6000.c (save_reg_p): Correct calls_eh_return
        case.  Match logic in rs6000_emit_prologue emitting pic_offset_table
        setup, simplifying ABI_V4 case to df_regs_ever_live_p.

diff --git a/gcc/config/rs6000/rs6000.c b/gcc/config/rs6000/rs6000.c
index cced90bb518..9933d4443f7 100644
--- a/gcc/config/rs6000/rs6000.c
+++ b/gcc/config/rs6000/rs6000.c
@@ -24013,18 +24013,26 @@ save_reg_p (int reg)
 
   if (reg == RS6000_PIC_OFFSET_TABLE_REGNUM && !TARGET_SINGLE_PIC_BASE)
     {
+      if (df_regs_ever_live_p (reg))
+       return true;
+
       /* When calling eh_return, we must return true for all the cases
         where conditional_register_usage marks the PIC offset reg
-        call used.  */
+        call used or fixed.  */
+      if (crtl->calls_eh_return
+         && ((DEFAULT_ABI == ABI_V4 && flag_pic)
+             || (DEFAULT_ABI == ABI_DARWIN && flag_pic)
+             || (TARGET_TOC && TARGET_MINIMAL_TOC)))
+       return true;
+
       if (TARGET_TOC && TARGET_MINIMAL_TOC
-         && (crtl->calls_eh_return
-             || df_regs_ever_live_p (reg)
-             || !constant_pool_empty_p ()))
+         && !constant_pool_empty_p ())
        return true;
 
-      if ((DEFAULT_ABI == ABI_V4 || DEFAULT_ABI == ABI_DARWIN)
+      if (DEFAULT_ABI == ABI_DARWIN
          && flag_pic && crtl->uses_pic_offset_table)
        return true;
+      return false;
     }
 
   return !call_used_regs[reg] && df_regs_ever_live_p (reg);

-- 
Alan Modra
Australia Development Lab, IBM