[gcc(refs/vendors/vrull/heads/for-upstream)] RISC-V: bitmanip: add splitter to use bexti for "(a & (1 << BIT_NO)) ? 0 : -1"

[Philipp Tomsich <ptomsich@gcc.gnu.org>]

https://gcc.gnu.org/g:d67fd1ea93402c5ec736c141955173e8a3cee966

commit d67fd1ea93402c5ec736c141955173e8a3cee966
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.
    
    Series-to: gcc-patches@gcc.gnu.org
    Series-cc: Palmer Dabbelt <palmer@rivosinc.com>
    Series-cc: Vineet Gupta <vineetg@rivosinc.com>
    Series-cc: Christoph Muellner <christoph.muellner@vrull.eu>
    Series-cc: Kito Cheng <kito.cheng@gmail.com>
    Series-cc: Jeff Law <jlaw@ventanamicro.com>

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

diff --git a/gcc/config/riscv/bitmanip.md b/gcc/config/riscv/bitmanip.md
index 48c249f2a68..1d603906cb2 100644
--- a/gcc/config/riscv/bitmanip.md
+++ b/gcc/config/riscv/bitmanip.md
@@ -402,3 +402,16 @@
   "TARGET_ZBS && UINTVAL (operands[2]) < GET_MODE_BITSIZE (<MODE>mode)"
   "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" } } */