PR rtl-optimization/109476: Use ZERO_EXTEND instead of zeroing a SUBREG.

This patch fixes PR rtl-optimization/109476, which is a code quality
regression affecting AVR.  The cause is that the lower-subreg pass is
sometimes overly aggressive, lowering the LSHIFTRT below:

(insn 7 4 8 2 (set (reg:HI 51)
        (lshiftrt:HI (reg/v:HI 49 [ b ])
            (const_int 8 [0x8]))) "t.ii":4:36 557 {lshrhi3}
     (nil))

into a pair of QImode SUBREG assignments:

(insn 19 4 20 2 (set (subreg:QI (reg:HI 51) 0)
        (reg:QI 54 [ b+1 ])) "t.ii":4:36 86 {movqi_insn_split}
     (nil))
(insn 20 19 8 2 (set (subreg:QI (reg:HI 51) 1)
        (const_int 0 [0])) "t.ii":4:36 86 {movqi_insn_split}
     (nil))

but this idiom, SETs of SUBREGs, interferes with combine's ability
to associate/fuse instructions.  The solution, on targets that
have a suitable ZERO_EXTEND (i.e. where the lower-subreg pass
wouldn't itself split a ZERO_EXTEND, so "splitting_zext" is false),
is to split/lower LSHIFTRT to a ZERO_EXTEND.

To answer Richard's question in comment gcc-mirror#10 of the bugzilla PR,
the function resolve_shift_zext is called with one of four RTX
codes, ASHIFTRT, LSHIFTRT, ZERO_EXTEND and ASHIFT, but only with
LSHIFTRT can the setting of low_part and high_part SUBREGs be
replaced by a ZERO_EXTEND.  For ASHIFTRT, we require a sign
extension, so don't set the high_part to zero; if we're splitting
a ZERO_EXTEND then it doesn't make sense to replace it with a
ZERO_EXTEND, and for ASHIFT we've played games to swap the
high_part and low_part SUBREGs, so that we assign the low_part
to zero (for double word shifts by greater than word size bits).

2023-04-28  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR rtl-optimization/109476
	* lower-subreg.cc: Include explow.h for force_reg.
	(find_decomposable_shift_zext): Pass an additional SPEED_P argument.
	If decomposing a suitable LSHIFTRT and we're not splitting
	ZERO_EXTEND (based on the current SPEED_P), then use a ZERO_EXTEND
	instead of setting a high part SUBREG to zero, which helps combine.
	(decompose_multiword_subregs): Update call to resolve_shift_zext.

gcc/testsuite/ChangeLog
	PR rtl-optimization/109476
	* gcc.target/avr/mmcu/pr109476.c: New test case.

gcc/lower-subreg.cc

@@ -37,6 +37,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "cfgbuild.h"
 #include "dce.h"
 #include "expr.h"
+#include "explow.h"
 #include "tree-pass.h"
 #include "lower-subreg.h"
 #include "rtl-iter.h"
@@ -1299,11 +1300,12 @@ find_decomposable_shift_zext (rtx_insn *insn, bool speed_p)
 
 /* Decompose a more than word wide shift (in INSN) of a multiword
    pseudo or a multiword zero-extend of a wordmode pseudo into a move
-   and 'set to zero' insn.  Return a pointer to the new insn when a
-   replacement was done.  */
+   and 'set to zero' insn.  SPEED_P says whether we are optimizing
+   for speed or size, when checking if a ZERO_EXTEND is preferable.
+   Return a pointer to the new insn when a replacement was done.  */
 
 static rtx_insn *
-resolve_shift_zext (rtx_insn *insn)
+resolve_shift_zext (rtx_insn *insn, bool speed_p)
 {
   rtx set;
   rtx op;
@@ -1378,14 +1380,29 @@ resolve_shift_zext (rtx_insn *insn)
 				dest_reg, GET_CODE (op) != ASHIFTRT);
     }
 
-  if (dest_reg != src_reg)
-    emit_move_insn (dest_reg, src_reg);
-  if (GET_CODE (op) != ASHIFTRT)
-    emit_move_insn (dest_upper, CONST0_RTX (word_mode));
-  else if (INTVAL (XEXP (op, 1)) == 2 * BITS_PER_WORD - 1)
-    emit_move_insn (dest_upper, copy_rtx (src_reg));
+  /* Consider using ZERO_EXTEND instead of setting DEST_UPPER to zero
+     if this is considered reasonable.  */
+  if (GET_CODE (op) == LSHIFTRT
+      && GET_MODE (op) == twice_word_mode
+      && REG_P (SET_DEST (set))
+      && !choices[speed_p].splitting_zext)
+    {
+      rtx tmp = force_reg (word_mode, copy_rtx (src_reg));
+      tmp = simplify_gen_unary (ZERO_EXTEND, twice_word_mode, tmp, word_mode);
+      emit_move_insn (SET_DEST (set), tmp);
+    }
   else
-    emit_move_insn (dest_upper, upper_src);
+    {
+      if (dest_reg != src_reg)
+	emit_move_insn (dest_reg, src_reg);
+      if (GET_CODE (op) != ASHIFTRT)
+	emit_move_insn (dest_upper, CONST0_RTX (word_mode));
+      else if (INTVAL (XEXP (op, 1)) == 2 * BITS_PER_WORD - 1)
+	emit_move_insn (dest_upper, copy_rtx (src_reg));
+      else
+	emit_move_insn (dest_upper, upper_src);
+    }
+
   insns = get_insns ();
 
   end_sequence ();
@@ -1670,7 +1687,7 @@ decompose_multiword_subregs (bool decompose_copies)
 		    {
 		      rtx_insn *decomposed_shift;
 
-		      decomposed_shift = resolve_shift_zext (insn);
+		      decomposed_shift = resolve_shift_zext (insn, speed_p);
 		      if (decomposed_shift != NULL_RTX)
 			{
 			  insn = decomposed_shift;

gcc/testsuite/gcc.target/avr/mmcu/pr109476.c

@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-Os -mmcu=avrxmega3" } */
+
+unsigned short foo(unsigned char a, unsigned short b) {
+    return (unsigned char)((b >> 8) + 0) * a ;
+}
+
+/* { dg-final { scan-assembler-times "mul" 1 } } */
+/* { dg-final { scan-assembler-times "mov" 1 } } */
+/* { dg-final { scan-assembler-not "add" } } */
+/* { dg-final { scan-assembler-not "ldi" } } */