RISC-V: Disable -msave-restore for shared libraries.

This was noticed while trying to test -msave-restore support.  The
save/restore routines use the alternate return register t0/x5 which is
clobbered by the PLT header, so we can't use them in shared libraries.
This patch disables -msave-restore when -fpic (and -mplt), and emits a
warning if the user explicitly turned on -msave-restore.

	gcc/
	* config/riscv/riscv.c (riscv_option_override): If -msave-restore
	and -fpic and -mplt then disable -msave-restore and warn.

From-SVN: r275231

gcc/ChangeLog

+2019-08-30  Jim Wilson  <jimw@sifive.com>
+
+	* config/riscv/riscv.c (riscv_option_override): If -msave-restore
+	and -fpic and -mplt then disable -msave-restore and warn.
+
 2019-08-30  Martin Sebor  <msebor@redhat.com>
 
 	PR middle-end/91599

gcc/config/riscv/riscv.c

@@ -4636,6 +4636,16 @@ riscv_option_override (void)
     error ("%<-mriscv-attribute%> RISC-V ELF attribute requires GNU as 2.32"
 	   " [%<-mriscv-attribute%>]");
 #endif
+
+  /* The save-restore routines use t0 which is clobbered by the plt header,
+     so we can't use them when building shared libraries.  */
+  if (TARGET_SAVE_RESTORE && flag_pic && TARGET_PLT)
+    {
+      target_flags &= ~MASK_SAVE_RESTORE;
+      if (target_flags_explicit & MASK_SAVE_RESTORE)
+	warning (0, "%<-msave-restore%> disabled; not supported with PLT "
+		 "based shared libraries");
+    }
 }
 
 /* Implement TARGET_CONDITIONAL_REGISTER_USAGE.  */