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Re: [GCC][PATCH][Aarch64] Exploiting BFXIL when OR-ing two AND-operations with appropriate bitmasks


Hi all,

Here is an updated patch that does the following:

* Adds a new constraint in config/aarch64/constraints.md to check for a constant integer that is left consecutive. This addresses Richard Henderson's suggestion about combining the aarch64_is_left_consecutive call and the const_int match in the pattern.

* Merges the two patterns defined into one.

* Changes the pattern's type attribute to bfm.

* Improved the comment above the aarch64_is_left_consecutive implementation.

* Makes the pattern use the GPI iterator to accept smaller integer sizes (an extra test is added to check for this).

* Improves the tests in combine_bfxil.c to ensure they aren't optimised away and that they check for the pattern's correctness.

Below is a new changelog and the example given before.

Is this OK for trunk?

This patch adds an optimisation that exploits the AArch64 BFXIL instruction
when or-ing the result of two bitwise and operations with non-overlapping
bitmasks (e.g. (a & 0xFFFF0000) | (b & 0x0000FFFF)).

Example:

unsigned long long combine(unsigned long long a, unsigned long long b) {
  return (a & 0xffffffff00000000ll) | (b & 0x00000000ffffffffll);
}

void read(unsigned long long a, unsigned long long b, unsigned long long *c) {
  *c = combine(a, b);
}

When compiled with -O2, read would result in:

read:
  and   x5, x1, #0xffffffff
  and   x4, x0, #0xffffffff00000000
  orr   x4, x4, x5
  str   x4, [x2]
  ret

But with this patch results in:

read:
  mov    x4, x0
  bfxil    x4, x1, 0, 32
  str    x4, [x2]
  ret



Bootstrapped and regtested on aarch64-none-linux-gnu and aarch64-none-elf with no regressions.


gcc/
2018-07-11  Sam Tebbs  <sam.tebbs@arm.com>

        PR target/85628
        * config/aarch64/aarch64.md (*aarch64_bfxil):
        Define.
        * config/aarch64/constraints.md (Ulc): Define
        * config/aarch64/aarch64-protos.h (aarch64_is_left_consecutive):
        Define.
        * config/aarch64/aarch64.c (aarch64_is_left_consecutive): New function.

gcc/testsuite
2018-07-11  Sam Tebbs  <sam.tebbs@arm.com>

        PR target/85628
        * gcc.target/aarch64/combine_bfxil.c: New file.
        * gcc.target/aarch64/combine_bfxil_2.c: New file.


On 07/19/2018 02:02 PM, Sam Tebbs wrote:
Hi Richard,

Thanks for the feedback. I find that using "is_left_consecutive" is more descriptive than checking for it being a power of 2 - 1, since it describes the requirement (having consecutive ones from the MSB) more explicitly. I would be happy to change it though if that is the consensus.

I have addressed your point about just returning the string instead of using output_asm_insn and have changed it locally. I'll send an updated patch soon.


On 07/17/2018 02:33 AM, Richard Henderson wrote:
On 07/16/2018 10:10 AM, Sam Tebbs wrote:
+++ b/gcc/config/aarch64/aarch64.c
@@ -1439,6 +1439,14 @@ aarch64_hard_regno_caller_save_mode (unsigned regno, unsigned,
      return SImode;
  }
  +/* Implement IS_LEFT_CONSECUTIVE.  Check if an integer's bits are consecutive
+   ones from the MSB.  */
+bool
+aarch64_is_left_consecutive (HOST_WIDE_INT i)
+{
+  return (i | (i - 1)) == HOST_WIDE_INT_M1;
+}
+
...
+(define_insn "*aarch64_bfxil"
+  [(set (match_operand:DI 0 "register_operand" "=r")
+    (ior:DI (and:DI (match_operand:DI 1 "register_operand" "r")
+            (match_operand 3 "const_int_operand"))
+        (and:DI (match_operand:DI 2 "register_operand" "0")
+            (match_operand 4 "const_int_operand"))))]
+  "INTVAL (operands[3]) == ~INTVAL (operands[4])
+    && aarch64_is_left_consecutive (INTVAL (operands[4]))"
Better is to use a define_predicate to merge both that second test and the
const_int_operand.

(I'm not sure about the "left_consecutive" language either.
Isn't it more descriptive to say that op3 is a power of 2 minus 1?)

(define_predicate "pow2m1_operand"
   (and (match_code "const_int")
        (match_test "exact_pow2 (INTVAL(op) + 1) > 0")))

and use

   (match_operand:DI 3 "pow2m1_operand")

and then just the

   INTVAL (operands[3]) == ~INTVAL (operands[4])

test.

Also, don't omit the modes for the constants.
Also, there's no reason this applies only to DI mode;
use the GPI iterator and %<w> in the output template.

+    HOST_WIDE_INT op3 = INTVAL (operands[3]);
+    operands[3] = GEN_INT (ceil_log2 (op3));
+    output_asm_insn ("bfxil\\t%0, %1, 0, %3", operands);
+    return "";
You can just return the string that you passed to output_asm_insn.

+  }
+  [(set_attr "type" "bfx")]
The other aliases of the BFM insn use type "bfm";
"bfx" appears to be aliases of UBFM and SBFM.
Not that it appears to matter to the scheduling
descriptions, but it is inconsistent.


r~



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