[PATCH] AArch64: Allow invert for shift counts [PR 123792]

[Wilco Dijkstra]

Optimize 1 << (31 - x) into 1 << ~x. This fixes part of PR 123792.

Passes regress, OK for commit?

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 --git a/gcc/config/aarch64/aarch64.md b/gcc/config/aarch64/aarch64.md
index 
4445208bf92ce0e08b72fde3de0f6dbc238cac3b..eb26c291e398993c093ff6f30e91bc56575f7583
 100644
--- a/gcc/config/aarch64/aarch64.md
+++ b/gcc/config/aarch64/aarch64.md
@@ -6157,7 +6157,8 @@ (define_insn_and_split "*aarch64_<optab>_reg_minus<mode>3"
          (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)]
@@ -6167,7 +6168,10 @@ (define_insn_and_split 
"*aarch64_<optab>_reg_minus<mode>3"
     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 
0000000000000000000000000000000000000000..9e09aa31024fcb07330c3733595b6402d7771ccb
--- /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);
+}