Comments (6)
To give a concrete example, this Motoko
func upperMask(a : Word8) : Bool = a & (0x80 : Word8) != (0 : Word8);
compiles currently to this Wasm:
(func $upperMask (type 5) (param $clos i32) (param $a i32) (result i32)
local.get $a
i32.const -2147483648 // 0x80000000
i32.and
i32.eqz
i32.eqz
i32.const 1
i32.shl)
We could have (unverified)
(func $upperMask (type 5) (param $clos i32) (param $a i32) (result i32)
local.get $a
i32.const 31
i32.shr
i32.const 1
i32.shl)
from motoko-base.
Something like
func upperBitGe(a : Word8) : Bool = a >= (0x80 : Word8);
func upperBitGt(a : Word8) : Bool = a > (0x7F : Word8);
with upperBitGt
will likely not work out of the box:
(func $upperBitGt (type 5) (param $clos i32) (param $a i32) (result i32)
local.get $a
i32.const 2130706432 // 0x7f000000
i32.gt_u
i32.const 1
i32.shl)
(func $upperBitGe (type 5) (param $clos i32) (param $a i32) (result i32)
local.get $a
i32.const -2147483648 // 0x80000000
i32.ge_u
i32.const 1
i32.shl)
unless we insert redundant hint instructions into the Wasm:
(func $upperBitGt (type 5) (param $clos i32) (param $a i32) (result i32)
local.get $a
hint (i32.const 0xFF000000; i32.and) -- adds some info about the TOS: 24 lower bits are zeroed
i32.const 2130706432 // 0x7f000000
i32.gt_u
i32.const 1
i32.shl)
With such hints we can perform the i32.const 31; i32.shr
trick.
I envision the hint
meta-instruction (never emitted) to operate on top-of-stack asserting a semantic nop
. This part is probably not beginner-friendly, though.
from motoko-base.
this is already performing: double swapping of the eqz; eqz; if
--> if
if
legs.
from motoko-base.
Also note that after a 1-bit masking shr 31; shl 1
is just shr 30
. Free tagging! Of course that is a different optimisation, since it needs the and
to be preserved.
from motoko-base.
See motoko/gabor/peephole-177
for an implementation sketch (very WIP).
from motoko-base.
Another reduction path:
block (result i32) ;; label = @1
i32.const 1
end
drop
block
--> const
--> eliminated by drop
from motoko-base.
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