[gcc r13-4128] RISC-V: bitmanip: add splitter to use bexti for "(a & (1 << BIT_NO)) ? 0 : -1"

[gcc r13-4128] RISC-V: bitmanip: add splitter to use bexti for "(a & (1 << BIT_NO)) ? 0 : -1"

[Philipp Tomsich]

https://gcc.gnu.org/g:1957bedf29a1b2cc231972aba680fe80199d5498

commit r13-4128-g1957bedf29a1b2cc231972aba680fe80199d5498
Author: Philipp Tomsich <philipp.tomsich@vrull.eu>
Date:   Tue Nov 9 18:54:54 2021 +0100

    RISC-V: bitmanip: add splitter to use bexti for "(a & (1 << BIT_NO)) ? 0 : -1"
    
    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.

Diff:
---
 gcc/config/riscv/bitmanip.md               | 13 +++++++++++++
 gcc/testsuite/gcc.target/riscv/zbs-bexti.c |  2 +-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/gcc/config/riscv/bitmanip.md b/gcc/config/riscv/bitmanip.md
index fb062f6eb43..0dd6ebbdd0b 100644
--- a/gcc/config/riscv/bitmanip.md
+++ b/gcc/config/riscv/bitmanip.md
@@ -429,3 +429,16 @@
 				      (const_int 1)
 				      (match_dup 2)))
    (set (match_dup 0) (xor:X (match_dup 0) (const_int 1)))])
+
+;; 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
index c15098eb6cc..d7a89638f46 100644
--- a/gcc/testsuite/gcc.target/riscv/zbs-bexti.c
+++ b/gcc/testsuite/gcc.target/riscv/zbs-bexti.c
@@ -26,6 +26,6 @@ long bexti64_4(long a, char bitno)
 }
 
 /* { dg-final { scan-assembler-times "bexti\t" 4 } } */
-/* { dg-final { scan-assembler-times "xori\t|snez\t" 1 } } */
+/* { dg-final { scan-assembler-times "xori\t" 1 } } */
 /* { dg-final { scan-assembler-times "addi\t" 1 } } */
 /* { dg-final { scan-assembler-times "neg\t" 1 } } */