RISC-V: avoid splitting small constants in bcrli_nottwobits patterns

RISC-V: avoid splitting small constants in bcrli_nottwobits patterns

[Jivan Hakobyan]

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Hi all.

I have noticed that in the case when we try to clear two bits through a
small constant,
and ZBS is enabled then GCC split it into two "andi" instructions.
For example for the following C code:
  int foo(int a) {
    return a & ~ 0x101;
  }

GCC generates the following:
  foo:
     andi a0,a0,-2
     andi a0,a0,-257
     ret

but should be this one:
  foo:
     andi a0,a0,-258
     ret


This patch solves the mentioned issue.


-- 
With the best regards
Jivan Hakobyan

[-- Attachment #2: and_nottwobits.patch --]
[-- Type: text/x-patch, Size: 3064 bytes --]

RISC-V: avoid splitting small constant in *bclri<mode>_nottwobits
        and *bclridisi_nottwobit patterns

gcc/
        * config/riscv/bitmanip.md Updated predicats of
        bclri<mode>_nottwobits and bclridisi_nottwobits patterns
        * config/riscv/predicates.md (not_uimm_extra_bit_or_nottwobits):
        Adjust predicate to avoid splitting arith constants
        * config/riscv/predicates.md (const_nottwobits_not_arith_operand):
        New predicate

gcc/testsuite
        * gcc.target/riscv/zbs-bclri-nottwobits.c: New test.

diff --git a/gcc/config/riscv/bitmanip.md b/gcc/config/riscv/bitmanip.md
index 388ef662820..f3d29a466e7 100644
--- a/gcc/config/riscv/bitmanip.md
+++ b/gcc/config/riscv/bitmanip.md
@@ -507,7 +507,7 @@
 (define_insn_and_split "*bclri<mode>_nottwobits"
   [(set (match_operand:X 0 "register_operand" "=r")
 	(and:X (match_operand:X 1 "register_operand" "r")
-	       (match_operand:X 2 "const_nottwobits_operand" "i")))]
+	       (match_operand:X 2 "const_nottwobits_not_arith_operand" "i")))]
   "TARGET_ZBS && !paradoxical_subreg_p (operands[1])"
   "#"
   "&& reload_completed"
@@ -526,7 +526,7 @@
 (define_insn_and_split "*bclridisi_nottwobits"
   [(set (match_operand:DI 0 "register_operand" "=r")
 	(and:DI (match_operand:DI 1 "register_operand" "r")
-		(match_operand:DI 2 "const_nottwobits_operand" "i")))]
+		(match_operand:DI 2 "const_nottwobits_not_arith_operand" "i")))]
   "TARGET_64BIT && TARGET_ZBS
    && clz_hwi (~UINTVAL (operands[2])) > 33"
   "#"
diff --git a/gcc/config/riscv/predicates.md b/gcc/config/riscv/predicates.md
index 8654dbc5943..e5adf06fa25 100644
--- a/gcc/config/riscv/predicates.md
+++ b/gcc/config/riscv/predicates.md
@@ -366,6 +366,11 @@
   (and (match_code "const_int")
        (match_test "popcount_hwi (~UINTVAL (op)) == 2")))
 
+(define_predicate "const_nottwobits_not_arith_operand"
+  (and (match_code "const_int")
+       (and (not (match_operand 0 "arith_operand"))
+	    (match_operand 0 "const_nottwobits_operand"))))
+
 ;; A CONST_INT operand that consists of a single run of 32 consecutive
 ;; set bits.
 (define_predicate "consecutive_bits32_operand"
@@ -411,4 +416,4 @@
 (define_predicate "not_uimm_extra_bit_or_nottwobits"
   (and (match_code "const_int")
        (ior (match_operand 0 "not_uimm_extra_bit_operand")
-	    (match_operand 0 "const_nottwobits_operand"))))
+	    (match_operand 0 "const_nottwobits_not_arith_operand"))))
diff --git a/gcc/testsuite/gcc.target/riscv/zbs-bclri-nottwobits.c b/gcc/testsuite/gcc.target/riscv/zbs-bclri-nottwobits.c
new file mode 100644
index 00000000000..5a58e0a1185
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/zbs-bclri-nottwobits.c
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-march=rv64gc_zbs -mabi=lp64" } */
+/* { dg-skip-if "" { *-*-* } { "-O0" "-Og" } } */
+
+int and_two_bit(int idx) {
+    return idx & ~3;
+}
+
+int and_bclr_two_bit(int idx) {
+    return idx & ~(0x4001);
+}
+
+/* { dg-final { scan-assembler-times "\tandi\t" 2 } } */
+/* { dg-final { scan-assembler-times "\tbclri\t" 1 } } */

Re: RISC-V: avoid splitting small constants in bcrli_nottwobits patterns

[Jeff Law]

On 4/20/23 14:11, Jivan Hakobyan via Gcc-patches wrote:
> and_nottwobits.patch
> 
> RISC-V: avoid splitting small constant in *bclri<mode>_nottwobits
>          and *bclridisi_nottwobit patterns
> 
> gcc/
>          * config/riscv/bitmanip.md Updated predicats of
>          bclri<mode>_nottwobits and bclridisi_nottwobits patterns
>          * config/riscv/predicates.md (not_uimm_extra_bit_or_nottwobits):
>          Adjust predicate to avoid splitting arith constants
>          * config/riscv/predicates.md (const_nottwobits_not_arith_operand):
>          New predicate
> 
> gcc/testsuite
>          * gcc.target/riscv/zbs-bclri-nottwobits.c: New test.
Thanks.  I made trivial fixes to the ChangeLog entry and pushed this to 
the trunk.

jeff