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
Signed-off-by: Philipp Tomsich <philipp.toms...@vrull.eu>
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.
---
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 0ab9ffe3c0b..ea5dea13cfb 100644
--- a/gcc/config/riscv/bitmanip.md
+++ b/gcc/config/riscv/bitmanip.md
@@ -340,3 +340,16 @@ (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_ZBS"
+ [(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..99e3b58309c
--- /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 4
+
+long
+foo0 (long a)
+{
+ return (a & (1 << BIT_NO)) ? 0 : -1;
+}
+
+/* { dg-final { scan-assembler "bexti" } } */
+/* { dg-final { scan-assembler "addi" } } */
--
2.34.1
