[to-be-committed] [RISC-V] Use bext for extracting a bit into a SImode object

bext is defined as (src >> n) & 1.  With that formulation, particularly the
"&1" means the result is implicitly zero extended.  So we can safely use it on
SI objects for rv64 without the need to do any explicit extension.

This patch adds the obvious pattern and a few testcases.   I think one of the
tests is derived from coremark, the other two from spec2017.

This has churned through Ventana's CI system repeatedly since it was first
written.  Assuming pre-commit CI doesn't complain, I'll commit it on Raphael's
behalf later today or Monday.

gcc/
	* config/riscv/bitmanip.md (*bextdisi): New pattern.

gcc/testsuite

	* gcc.target/riscv/bext-ext.c: New test.

gcc/config/riscv/bitmanip.md

@@ -754,6 +754,18 @@
   "operands[1] = gen_lowpart (word_mode, operands[1]);"
   [(set_attr "type" "bitmanip")])
 
+;; The logical-and against 0x1 implicitly extends the result.   So we can treat
+;; an SImode bext as-if it's DImode without any explicit extension.
+(define_insn "*bextdisi"
+  [(set (match_operand:DI 0 "register_operand" "=r")
+    (and:DI (subreg:DI (lshiftrt:SI
+			 (match_operand:SI 1 "register_operand" "r")
+			 (match_operand:QI 2 "register_operand" "r")) 0)
+            (const_int 1)))]
+  "TARGET_64BIT && TARGET_ZBS"
+  "bext\t%0,%1,%2"
+  [(set_attr "type" "bitmanip")])
+
 ;; When performing `(a & (1UL << bitno)) ? 0 : -1` the combiner
 ;; usually has the `bitno` typed as X-mode (i.e. no further
 ;; zero-extension is performed around the bitno).

gcc/testsuite/gcc.target/riscv/bext-ext.c

@@ -0,0 +1,27 @@
+/* { dg-do compile } */
+/* { dg-options "-march=rv64gc_zbs -mabi=lp64" } */
+/* { dg-skip-if "" { *-*-* } { "-O0" "-Og" "-O1" } } */
+typedef unsigned long uint64_t;
+typedef unsigned int uint32_t;
+
+uint64_t bext1 (int dst, const uint32_t i)
+{
+  uint64_t checks = 1U;
+  checks &= dst >> i;
+  return checks;
+}
+
+int bext2 (int dst, int i_denom)
+{
+  dst = 1 & (dst >> i_denom);
+  return dst;
+}
+
+const uint32_t bext3 (uint32_t bit_count, uint32_t symbol)
+{
+  return (symbol >> bit_count) & 1;
+}
+
+/* { dg-final { scan-assembler-times "bext\t" 3 } } */
+/* { dg-final { scan-assembler-not "sllw\t"} } */
+/* { dg-final { scan-assembler-not "srlw\t"} } */