[Bug rtl-optimization/121198] [avr][hreg-constraints] error: 'asm' operand has impossible constraints or there are not enough registers

[gjl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121198

--- Comment #2 from Georg-Johann Lay <gjl at gcc dot gnu.org> ---
Created attachment 61955
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=61955&action=edit
hrc-bswapsi.diff: Proposed patch to get rig of hard regs in avr.md

This patch also runs into ICE.  It tries to get rid of hard-regs in avr.md's
bswapsi2.

unsigned long swap32 (unsigned long x)
{
    return __builtin_bswap32 (x);
}

$ avr-gcc -Os -S bswap.c 
bswap.c: In function 'swap32':
bswap.c:4:1: error: unable to find a register to spill
    4 | }
      | ^
bswap.c:4:1: error: this is the insn:
(insn 6 16 15 2 (set (reg:SI 48 [orig:43 <retval> ] [43])
        (bswap:SI (reg:SI 49 [orig:46 x ] [46]))) "bswap.c":3:12 1052
{bswapsi2}
     (expr_list:REG_DEAD (reg:SI 49 [orig:46 x ] [46])
        (nil)))
during RTL pass: reload
bswap.c:4:1: internal compiler error: in lra_split_hard_reg_for, at
lra-assigns.cc:1863

There are many more patterns with hard regs in the avr backend.  Most of them
are modelling reduced register footprints of libgcc functions that are
implemented in asm.