[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/zbs-ext.c: New test.

gcc/config/riscv/bitmanip.md

@@ -684,6 +684,23 @@
 }
 [(set_attr "type" "bitmanip")])
 
+;; An outer AND with a constant where bits 31..63 are 0 can be seen as
+;; a virtual zero extension from 31 to 64 bits.
+(define_split
+  [(set (match_operand:DI 0 "register_operand")
+    (and:DI (not:DI (subreg:DI
+		      (ashift:SI (const_int 1)
+				 (match_operand:QI 1 "register_operand")) 0))
+            (match_operand:DI 2 "arith_operand")))
+   (clobber (match_operand:DI 3 "register_operand"))]
+  "TARGET_64BIT && TARGET_ZBS
+   && clz_hwi (INTVAL (operands[2])) >= 33"
+  [(set (match_dup 3)
+        (match_dup 2))
+   (set (match_dup 0)
+	  (and:DI (rotate:DI (const_int -2) (match_dup 1))
+		  (match_dup 3)))])
+
 (define_insn "*binv<mode>"
   [(set (match_operand:X 0 "register_operand" "=r")
 	(xor:X (ashift:X (const_int 1)

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

@@ -0,0 +1,15 @@
+/* { 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 bclr (const uint32_t i)
+{
+  uint64_t checks = 10;
+  checks &= ~(1U << i);
+  return checks;
+}
+
+/* { dg-final { scan-assembler-times "bclr\t" 1 } } */
+/* { dg-final { scan-assembler-not "sllw\t"} } */