[gcc r16-7064] AArch64: Allow invert for shift counts [PR 123792]

[Wilco Dijkstra]

https://gcc.gnu.org/g:3cb7c85eebd985509eae977a45d081b9038ce6ec

commit r16-7064-g3cb7c85eebd985509eae977a45d081b9038ce6ec
Author: Wilco Dijkstra <wilco.dijkstra@arm.com>
Date:   Fri Jan 23 17:48:00 2026 +0000

    AArch64: Allow invert for shift counts [PR 123792]
    
    Optimize 1 << (31 - x) into 1 << ~x. This fixes part of PR 123792.
    
    gcc:
            PR target/123792
            * config/aarch64/aarch64.md (aarch64_<optab>_reg_minus<mode>3):
            Add support for invert in shift count.
    
    gcc/testsuite:
            PR target/123792
            * gcc.target/aarch64/pr123792.c: New test.

Diff:
---
 gcc/config/aarch64/aarch64.md               |  8 +++--
 gcc/testsuite/gcc.target/aarch64/pr123792.c | 53 +++++++++++++++++++++++++++++
 2 files changed, 59 insertions(+), 2 deletions(-)

diff --git a/gcc/config/aarch64/aarch64.md b/gcc/config/aarch64/aarch64.md
index 568e5814afc2..70a64a6c0ed8 100644
--- a/gcc/config/aarch64/aarch64.md
+++ b/gcc/config/aarch64/aarch64.md
@@ -6178,7 +6178,8 @@
 	  (match_operand:GPI 1 "register_operand" "r")
 	  (minus:QI (match_operand 2 "const_int_operand" "n")
 		    (match_operand:QI 3 "register_operand" "r"))))]
-  "INTVAL (operands[2]) == GET_MODE_BITSIZE (<MODE>mode)"
+  "INTVAL (operands[2]) == GET_MODE_BITSIZE (<MODE>mode)
+   || INTVAL (operands[2]) == GET_MODE_BITSIZE (<MODE>mode) - 1"
   "#"
   "&& true"
   [(const_int 0)]
@@ -6188,7 +6189,10 @@
     rtx tmp = (can_create_pseudo_p () ? gen_reg_rtx (SImode)
 	       : gen_lowpart (SImode, operands[0]));
 
-    emit_insn (gen_negsi2 (tmp, subreg_tmp));
+    if (INTVAL (operands[2]) == GET_MODE_BITSIZE (<MODE>mode))
+      emit_insn (gen_negsi2 (tmp, subreg_tmp));
+    else
+      emit_insn (gen_one_cmplsi2 (tmp, subreg_tmp));
 
     rtx and_op = gen_rtx_AND (SImode, tmp,
 			      GEN_INT (GET_MODE_BITSIZE (<MODE>mode) - 1));
diff --git a/gcc/testsuite/gcc.target/aarch64/pr123792.c b/gcc/testsuite/gcc.target/aarch64/pr123792.c
new file mode 100644
index 000000000000..9e09aa31024f
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/pr123792.c
@@ -0,0 +1,53 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+/* { dg-final { check-function-bodies "**" "" "" } } */
+
+#include <stdint.h>
+
+/*
+** f1:
+**	mvn	w1, w1
+**	lsl	w0, w0, w1
+**	ret
+*/
+
+int f1 (int x, int n)
+{
+  return x << (31 - n);
+}
+
+/*
+** f2:
+**	mvn	w1, w1
+**	asr	w0, w0, w1
+**	ret
+*/
+
+int f2 (int x, int n)
+{
+  return x >> (31 - n);
+}
+
+/*
+** f3:
+**	mvn	w1, w1
+**	lsr	x0, x0, x1
+**	ret
+*/
+
+unsigned long f3 (unsigned long long x, int n)
+{
+  return x >> (63 - n);
+}
+
+/*
+** f4:
+**	mvn	w1, w1
+**	lsl	x0, x0, x1
+**	ret
+*/
+
+long f4 (long x, int n)
+{
+  return x << (63 - n);
+}