[gcc r14-2851] PR rtl-optimization/110587: Reduce useless moves in compile-time hog.

[gcc r14-2851] PR rtl-optimization/110587: Reduce useless moves in compile-time hog.

[Roger Sayle]

https://gcc.gnu.org/g:095eb138f736d94dabf9a07a6671bd351be0e66a

commit r14-2851-g095eb138f736d94dabf9a07a6671bd351be0e66a
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Fri Jul 28 09:39:46 2023 +0100

    PR rtl-optimization/110587: Reduce useless moves in compile-time hog.
    
    This patch is one of a 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 moves 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).
    
    2023-07-28  Roger Sayle  <roger@nextmovesoftware.com>
                Richard Biener  <rguenther@suse.de>
    
    gcc/ChangeLog
            PR middle-end/28071
            PR rtl-optimization/110587
            * expr.cc (emit_group_load_1): Simplify logic for calling
            force_reg on ORIG_SRC, to avoid making a copy if the source
            is already in a pseudo register.

Diff:
---
 gcc/expr.cc | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/gcc/expr.cc b/gcc/expr.cc
index fff09dc9951..174f8acb269 100644
--- a/gcc/expr.cc
+++ b/gcc/expr.cc
@@ -2622,16 +2622,11 @@ emit_group_load_1 (rtx *tmps, rtx dst, rtx orig_src, tree type,
 	 be loaded directly into the destination.  */
       src = orig_src;
       if (!MEM_P (orig_src)
-	  && (!CONSTANT_P (orig_src)
-	      || (GET_MODE (orig_src) != mode
-		  && GET_MODE (orig_src) != VOIDmode)))
+	  && (!REG_P (orig_src) || HARD_REGISTER_P (orig_src))
+	  && !CONSTANT_P (orig_src))
 	{
-	  if (GET_MODE (orig_src) == VOIDmode)
-	    src = gen_reg_rtx (mode);
-	  else
-	    src = gen_reg_rtx (GET_MODE (orig_src));
-
-	  emit_move_insn (src, orig_src);
+	  gcc_assert (GET_MODE (orig_src) != VOIDmode);
+	  src = force_reg (GET_MODE (orig_src), orig_src);
 	}
 
       /* Optimize the access just a bit.  */