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Allow muxing / cmov on bools

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Mamy André-Ratsimbazafy 2020-02-09 17:49:38 +01:00
parent 153e898263
commit 514715dff0
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1 changed files with 29 additions and 16 deletions
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45
constantine/primitives.nim
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@ -85,9 +85,9 @@ template `not`*[T: Ct](x: T): T =
T(not T.T(x)) T(not T.T(x))
func `+`*[T: Ct](x, y: T): T {.magic: "AddU".} func `+`*[T: Ct](x, y: T): T {.magic: "AddU".}
func `+=`*[T: Ct](x: var T, y: T): T {.magic: "Inc".} func `+=`*[T: Ct](x: var T, y: T) {.magic: "Inc".}
func `-`*[T: Ct](x, y: T): T {.magic: "SubU".} func `-`*[T: Ct](x, y: T): T {.magic: "SubU".}
func `-=`*[T: Ct](x: var T, y: T): T {.magic: "Dec".} func `-=`*[T: Ct](x: var T, y: T) {.magic: "Dec".}
func `shr`*[T: Ct](x: T, y: SomeInteger): T {.magic: "ShrI".} func `shr`*[T: Ct](x: T, y: SomeInteger): T {.magic: "ShrI".}
func `shl`*[T: Ct](x: T, y: SomeInteger): T {.magic: "ShlI".} func `shl`*[T: Ct](x: T, y: SomeInteger): T {.magic: "ShlI".}
@ -130,20 +130,6 @@ func `not`*(ctl: CTBool): CTBool =
func `and`*(x, y: CTBool): CTBool {.magic: "BitandI".} func `and`*(x, y: CTBool): CTBool {.magic: "BitandI".}
func `or`*(x, y: CTBool): CTBool {.magic: "BitorI".} func `or`*(x, y: CTBool): CTBool {.magic: "BitorI".}
template mux*[T: Ct](ctl: CTBool[T], x, y: T): T =
## Multiplexer / selector
## Returns x if ctl is true
## else returns y
## So equivalent to ctl? x: y
y xor (-T(ctl) and (x xor y))
# TODO verify assembly generated
# as mentioned in https://cryptocoding.net/index.php/Coding_rules
# the alternative `(x and ctl) or (y and -ctl)`
# is optimized into a branch by Clang :/
# TODO: assembly fastpath for conditional mov
func noteq[T: Ct](x, y: T): CTBool[T] = func noteq[T: Ct](x, y: T): CTBool[T] =
const msb = T.sizeof * 8 - 1 const msb = T.sizeof * 8 - 1
let z = x xor y let z = x xor y
@ -162,6 +148,33 @@ func `<`*[T: Ct](x, y: T): CTBool[T] =
func `<=`*[T: Ct](x, y: T): CTBool[T] = func `<=`*[T: Ct](x, y: T): CTBool[T] =
not(y < x) not(y < x)
func `==`*(x, y: CTBool): CTBool =
(type result)(x.undistinct == y.undistinct)
func `xor`*(x, y: CTBool): CTBool =
(type result)(x.undistinct.noteq(y.undistinct))
template mux*[T: Ct](ctl: CTBool[T], x, y: T): T =
## Multiplexer / selector
## Returns x if ctl is true
## else returns y
## So equivalent to ctl? x: y
y xor (-T(ctl) and (x xor y))
# TODO verify assembly generated
# as mentioned in https://cryptocoding.net/index.php/Coding_rules
# the alternative `(x and ctl) or (y and -ctl)`
# is optimized into a branch by Clang :/
# TODO: assembly fastpath for conditional mov
template mux*[T: CTBool](ctl: CTBool, x, y: T): T =
## Multiplexer / selector
## Returns x if ctl is true
## else returns y
## So equivalent to ctl? x: y
T(T.T(y) xor (-T.T(ctl) and T.T(x xor y)))
# ############################################################ # ############################################################
# #
# Workaround system.nim `!=` template # Workaround system.nim `!=` template