[PATCH v1 6/8] RISC-V: bitmanip: add splitter to use bexti for "(a & (1 << BIT_NO)) ? 0 : -1"

Philipp Tomsich philipp.tomsich@vrull.eu
Thu Nov 11 14:10:18 GMT 2021


Consider creating a polarity-reversed mask from a set-bit (i.e., if
the bit is set, produce all-ones; otherwise: all-zeros).  Using Zbb,
this can be expressed as bexti, followed by an addi of minus-one.  To
enable the combiner to discover this opportunity, we need to split the
canonical expression for "(a & (1 << BIT_NO)) ? 0 : -1" into a form
combinable into bexti.

Consider the function:
    long f(long a)
    {
      return (a & (1 << BIT_NO)) ? 0 : -1;
    }
This produces the following sequence prior to this change:
	andi	a0,a0,16
	seqz	a0,a0
	neg	a0,a0
	ret
Following this change, it results in:
	bexti	a0,a0,4
	addi	a0,a0,-1
	ret

gcc/ChangeLog:

	* config/riscv/bitmanip.md: Add a splitter to generate
          polarity-reversed masks from a set bit using bexti + addi.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/zbs-bexti.c: New test.

Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
---

 gcc/config/riscv/bitmanip.md               | 13 +++++++++++++
 gcc/testsuite/gcc.target/riscv/zbs-bexti.c | 14 ++++++++++++++
 2 files changed, 27 insertions(+)
 create mode 100644 gcc/testsuite/gcc.target/riscv/zbs-bexti.c

diff --git a/gcc/config/riscv/bitmanip.md b/gcc/config/riscv/bitmanip.md
index 178d1ca0e4b..9e10280e306 100644
--- a/gcc/config/riscv/bitmanip.md
+++ b/gcc/config/riscv/bitmanip.md
@@ -367,3 +367,16 @@ (define_insn "*bexti"
   "TARGET_ZBS"
   "bexti\t%0,%1,%2"
   [(set_attr "type" "bitmanip")])
+
+;; We can create a polarity-reversed mask (i.e. bit N -> { set = 0, clear = -1 })
+;; using a bext(i) followed by an addi instruction.
+;; This splits the canonical representation of "(a & (1 << BIT_NO)) ? 0 : -1".
+(define_split
+  [(set (match_operand:GPR 0 "register_operand")
+       (neg:GPR (eq:GPR (zero_extract:GPR (match_operand:GPR 1 "register_operand")
+                                          (const_int 1)
+                                          (match_operand 2))
+                        (const_int 0))))]
+  "TARGET_ZBB"
+  [(set (match_dup 0) (zero_extract:GPR (match_dup 1) (const_int 1) (match_dup 2)))
+   (set (match_dup 0) (plus:GPR (match_dup 0) (const_int -1)))])
diff --git a/gcc/testsuite/gcc.target/riscv/zbs-bexti.c b/gcc/testsuite/gcc.target/riscv/zbs-bexti.c
new file mode 100644
index 00000000000..d02c3f7a98d
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/zbs-bexti.c
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-march=rv64gc_zbs -mabi=lp64 -O2" } */
+
+/* bexti */
+#define BIT_NO  27
+
+long
+foo0 (long a)
+{
+  return (a & (1 << BIT_NO)) ? 0 : -1;
+}
+
+/* { dg-final { scan-assembler "bexti" } } */
+/* { dg-final { scan-assembler "addi" } } */
-- 
2.32.0



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