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Don't use memcpy / memmove (#57)

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Mamy Ratsimbazafy 2020-06-19 20:29:11 +02:00 committed by GitHub
parent 608d26e497
commit b509eeaeb6
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1 changed files with 28 additions and 11 deletions
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39
constantine/arithmetic/limbs_modular.nim
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@ -200,6 +200,23 @@ template `[]`*(v: LimbsViewMut, limbIdx: int): var SecretWord =
template `[]=`*(v: LimbsViewMut, limbIdx: int, val: SecretWord) =
LimbsView(v)[limbIdx] = val
# Copy
# ------------------------------------------------------------
func copyWords(
a: LimbsViewMut, startA: int,
b: LimbsViewAny, startB: int,
numWords: int
) =
## Copy a slice of B into A. This properly deals
## with overlaps when A and B are slices of the same buffer
if startA > startB:
for i in countdown(numWords-1, 0):
a[startA+i] = b[startB+i]
else:
for i in 0 ..< numWords:
a[startA+i] = b[startB+i]
# Type-erased add-sub
# ------------------------------------------------------------
@ -260,20 +277,20 @@ func shlAddMod_estimate(a: LimbsViewMut, aLen: int,
v[`v Len` - limbIdxFromEnd.int]
# ----------------------------------------------------------------------
# Assuming 64-bit words
let hi = a[^1] # Save the high word to detect carries
let R = mBits and (WordBitWidth - 1) # R = mBits mod 64
# Assuming 64-bit words
let hi = a[^1] # Save the high word to detect carries
let R = mBits and (WordBitWidth - 1) # R = mBits mod 64
var a0, a1, m0: SecretWord
if R == 0: # If the number of mBits is a multiple of 64
a0 = a[^1] #
moveMem(a[1].addr, a[0].addr, (aLen-1) * SecretWord.sizeof) # we can just shift words
a[0] = c # and replace the first one by c
if R == 0: # If the number of mBits is a multiple of 64
a0 = a[^1] #
copyWords(a, 1, a, 0, aLen-1) # we can just shift words
a[0] = c # and replace the first one by c
a1 = a[^1]
m0 = M[^1]
else: # Else: need to deal with partial word shifts at the edge.
else: # Else: need to deal with partial word shifts at the edge.
a0 = (a[^1] shl (WordBitWidth-R)) or (a[^2] shr R)
moveMem(a[1].addr, a[0].addr, (aLen-1) * SecretWord.sizeof)
copyWords(a, 1, a, 0, aLen-1)
a[0] = c
a1 = (a[^1] shl (WordBitWidth-R)) or (a[^2] shr R)
m0 = (M[^1] shl (WordBitWidth-R)) or (M[^2] shr R)
@ -365,7 +382,7 @@ func reduce(r: LimbsViewMut,
# if a uses less bits than the modulus,
# it is guaranteed < modulus.
# This relies on the precondition that the modulus uses all declared bits
copyMem(r[0].addr, a[0].unsafeAddr, aLen * sizeof(SecretWord))
copyWords(r, 0, a, 0, aLen)
for i in aLen ..< mLen:
r[i] = Zero
else:
@ -373,7 +390,7 @@ func reduce(r: LimbsViewMut,
# we can copy modulus.limbs-1 words
# and modular shift-left-add the rest
let aOffset = aLen - mLen
copyMem(r[0].addr, a[aOffset+1].unsafeAddr, (mLen-1) * sizeof(SecretWord))
copyWords(r, 0, a, aOffset+1, mLen-1)
r[rLen - 1] = Zero
# Now shift-left the copied words while adding the new word modulo M
for i in countdown(aOffset, 0):