[Bug rtl-optimization/70961] Regrename ignores preferred_rename_class

[ebotcazou at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70961

--- Comment #2 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
> The super_class has nothing to do with the class that is searched for
> renaming registers though, it's just the info passed to the back-end to
> compute this class.  For example, on the ARM, the preferred_class will be
> LO_REGS for any chain of GENERAL_REGS.

As a matter of fact, that might not be true, something like

Index: config/arm/arm.c
===================================================================
--- config/arm/arm.c    (revision 235858)
+++ config/arm/arm.c    (working copy)
@@ -27721,9 +27721,9 @@ static reg_class_t
 arm_preferred_rename_class (reg_class_t rclass)
 {
   /* Thumb-2 instructions using LO_REGS may be smaller than instructions
-     using GENERIC_REGS.  During register rename pass, we prefer LO_REGS,
+     using GENERAL_REGS.  During register rename pass, we prefer LO_REGS,
      and code size can be reduced.  */
-  if (TARGET_THUMB2 && rclass == GENERAL_REGS)
+  if (TARGET_THUMB2 && reg_class_subset_p (rclass, GENERAL_REGS))
     return LO_REGS;
   else
     return NO_REGS;

could be needed.