diff mbox series

[to-be-committed,RISC-V] Improve single inverted bit extraction

Message ID bc964541-323d-47de-ac8e-e271c9fc99c2@ventanamicro.com
State New
Headers show
Series [to-be-committed,RISC-V] Improve single inverted bit extraction | expand

Commit Message

Jeff Law May 12, 2024, 2:24 p.m. UTC 
So the first time I sent this, I attached the wrong patch.  As a result 
the CI system wasn't happy.

The second time I sent the right patch, but I don't see evidence the CI 
system ran the correct patch through.  So I'm just starting over ;-)

--

So this patch fixes a minor code generation inefficiency that (IIRC) the
RAU team discovered a while ago in spec.

If we want the inverted value of a single bit we can use bext to extract
the bit, then seq to invert the value (if viewed as a 0/1 truth value).

The RTL is fairly convoluted, but it's basically a right shift to get
the bit into position, bitwise-not then masking off all but the low bit.
So it's a 3->2 combine, hidden by the fact that and-not is a
define_insn_and_split, so it actually looks like a 2->2 combine.

We've run this through Ventana's internal CI (which includes
zba_zbb_zbs) and I've run it in my own tester (rv64gc, rv32gcv).  I'll
wait for the upstream CI to finish with positive results before pushing.

Jeff
* config/riscv/riscv.cc (riscv_build_integer_1): Recognize cases where
	we can use shNadd to improve constant synthesis.
	(riscv_move_integer): Handle code generation for shNadd.

gcc/testsuite
	* gcc.target/riscv/synthesis-1.c: Also count shNadd instructions.
	* gcc.target/riscv/synthesis-3.c: New test.
diff mbox series

Patch

diff --git a/gcc/config/riscv/bitmanip.md b/gcc/config/riscv/bitmanip.md
index d76a72d30e0..cf2fa04d4c4 100644
--- a/gcc/config/riscv/bitmanip.md
+++ b/gcc/config/riscv/bitmanip.md
@@ -711,6 +711,30 @@  (define_insn "*bext<mode>"
   "bext\t%0,%1,%2"
   [(set_attr "type" "bitmanip")])
 
+;; This is a bext followed by a seqz.  Normally this would be a 3->2 split
+;; But the and-not pattern with a constant operand is a define_insn_and_split,
+;; so this looks like a 2->2 split, which combine rejects.  So implement it
+;; as a define_insn_and_split as well.
+(define_insn_and_split "*bextseqzdisi"
+  [(set (match_operand:DI 0 "register_operand" "=r")
+	(and:DI
+	  (not: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"
+  "#"
+  "&& 1"
+  [(set (match_dup 0) (and:DI (subreg:DI
+			        (lshiftrt:SI (match_dup 1)
+					     (match_dup 2)) 0)
+			      (const_int 1)))
+   (set (match_dup 0) (eq:DI (match_dup 0) (const_int 0)))]
+  ""
+  [(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).
diff --git a/gcc/testsuite/gcc.target/riscv/zbs-bext-2.c b/gcc/testsuite/gcc.target/riscv/zbs-bext-2.c
new file mode 100644
index 00000000000..53f47dc3afe
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/zbs-bext-2.c
@@ -0,0 +1,15 @@ 
+/* { dg-do compile } */
+/* { dg-options "-march=rv64gc_zbs -mabi=lp64" } */
+/* { dg-skip-if "" { *-*-* } { "-O0" } } */
+
+
+_Bool match(const int ch, int fMap) {
+    return ((fMap & (1<<(ch))) == 0);
+}
+
+
+/* { dg-final { scan-assembler-times "bext\t" 1 } } */
+/* { dg-final { scan-assembler-times "seqz\t" 1 } } */
+/* { dg-final { scan-assembler-not "sraw\t" } } */
+/* { dg-final { scan-assembler-not "not\t" } } */
+/* { dg-final { scan-assembler-not "andi\t" } } */