This patch is the third in series of fixes for PR rtl-optimization/110587,
a compile-time regression with -O0, that attempts to address the underlying
cause.  As noted previously, the pathological test case pr28071.c contains
a large number of useless register-to-register moves that can produce
quadratic behaviour (in LRA).  These move are generated during RTL
expansion in emit_group_load_1, where the middle-end attempts to simplify
the source before calling extract_bit_field.  This is reasonable if the
source is a complex expression (from before the tree-ssa optimizers), or
a SUBREG, or a hard register, but it's not particularly useful to copy
a pseudo register into a new pseudo register.  This patch eliminates that
redundancy.

The -fdump-tree-expand for pr28071.c compiled with -O0 currently contains
777K lines, with this patch it contains 717K lines, i.e. saving about 60K
lines (admittedly of debugging text output, but it makes the point).


This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32}
with no new failures.  Ok for mainline?

As always, I'm happy to revert this change quickly if there's a problem,
and investigate why this additional copy might (still) be needed on other
non-x86 targets.


2023-07-25  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        PR middle-end/28071
        PR rtl-optimization/110587
        * expr.cc (emit_group_load_1): Avoid copying a pseudo register into
        a new pseudo register, i.e. only copy hard regs into a new pseudo.


Thanks in advance,
Roger
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