[Bug rtl-optimization/87948] New: LRA register allocator does not support HARD_REGNO_CALLER_SAVE_MODE returning VOIDmode

["meissner at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87948

            Bug ID: 87948
           Summary: LRA register allocator does not support
                    HARD_REGNO_CALLER_SAVE_MODE returning VOIDmode
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: meissner at gcc dot gnu.org
  Target Milestone: ---

The documentation for HARD_REGNO_CALLER_SAVE_MODE states:

@defmac HARD_REGNO_CALLER_SAVE_MODE (@var{regno}, @var{nregs})
A C expression specifying which mode is required for saving @var{nregs}
of a pseudo-register in call-clobbered hard register @var{regno}.  If
@var{regno} is unsuitable for caller save, @code{VOIDmode} should be
returned.  For most machines this macro need not be defined since GCC
will select the smallest suitable mode.
@end defmac

I was trying to prevent the GCC compiler from saving and restoring CR registers
across calls, so I defined HARD_REGNO_CALLER_SAVE_MODE as:

/* When setting up caller-save slots (MODE == VOIDmode) ensure we allocate      
   enough space to account for vectors in FP regs.  However, TFmode/TDmode      
   should not use VSX instructions to do a caller save.  Return VOIDmode        
   for the CR registers so that we don't try to save them across calls.  */
#define HARD_REGNO_CALLER_SAVE_MODE(REGNO, NREGS, MODE)                 \
  (CR_REGNO_P (REGNO)                                                   \
   ? VOIDmode                                                           \
   : (NREGS) <= rs6000_hard_regno_nregs[MODE][REGNO]                    \
   ? (MODE)                                                             \
   : TARGET_VSX                                                         \
     && ((MODE) == VOIDmode || ALTIVEC_OR_VSX_VECTOR_MODE (MODE))       \
     && FP_REGNO_P (REGNO)                                              \
   ? V2DFmode                                                           \
   : FLOAT128_IBM_P (MODE) && FP_REGNO_P (REGNO)                        \
   ? DFmode                                                             \
   : (MODE) == TDmode && FP_REGNO_P (REGNO)                             \
   ? DImode                                                             \
   : choose_hard_reg_mode ((REGNO), (NREGS), false))

However, when I try to build the compiler (without bootstrap initially), I get
the following error:

during RTL pass: reload
/home/meissner/fsf-src/test/libbacktrace/elf.c: In function 'elf_add':
/home/meissner/fsf-src/test/libbacktrace/elf.c:3211:1: internal compiler error:
in lra_create_new_reg_with_unique_value, at lra.c:189
 3211 | }
      | ^
0x107bba3f lra_create_new_reg_with_unique_value(machine_mode, rtx_def*,
reg_class, char const*)
        /home/meissner/fsf-src/test/gcc/lra.c:189
0x107bbabf lra_create_new_reg(machine_mode, rtx_def*, reg_class, char const*)
        /home/meissner/fsf-src/test/gcc/lra.c:229
0x107ce00b split_reg
        /home/meissner/fsf-src/test/gcc/lra-constraints.c:5566
0x107d5aa3 inherit_in_ebb
        /home/meissner/fsf-src/test/gcc/lra-constraints.c:6568
0x107d5aa3 lra_inheritance()
        /home/meissner/fsf-src/test/gcc/lra-constraints.c:6639
0x107c0f37 lra(_IO_FILE*)
        /home/meissner/fsf-src/test/gcc/lra.c:2486
0x1075abeb do_reload
        /home/meissner/fsf-src/test/gcc/ira.c:5469
0x1075abeb execute
        /home/meissner/fsf-src/test/gcc/ira.c:5653
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.
Makefile:820: recipe for target 'elf.lo' failed
make[3]: *** [elf.lo] Error 1
make[3]: Leaving directory
'/home/meissner/fsf-build-ppc64le/test/powerpc64le-unknown-linux-gnu/libbacktrace'
Makefile:700: recipe for target 'all' failed
make[2]: *** [all] Error 2
make[2]: Leaving directory
'/home/meissner/fsf-build-ppc64le/test/powerpc64le-unknown-linux-gnu/libbacktrace'
Makefile:13084: recipe for target 'all-target-libbacktrace' failed
make[1]: *** [all-target-libbacktrace] Error 2
make[1]: Leaving directory '/home/meissner/fsf-build-ppc64le/test'
Makefile:939: recipe for target 'all' failed
make: *** [all] Error 2