Add extended precision multiplication

constantine/bigints.nim

@@ -50,12 +50,15 @@ type
   BigInt*[bits: static int] = object
     limbs*: array[bits.words_required, Limb]
 
-const highLimb* = (not Ct[uint64](0)) shr 1
+const HighLimb* = (not Ct[uint64](0)) shr 1
   ## This represents 0x7F_FF_FF_FF__FF_FF_FF_FF
   ## also 0b0111...1111
   ## This biggest representable number in our limbs.
   ## i.e. The most significant bit is never set at the end of each function
 
+template `[]`*(a: Bigint, idx: int): Limb =
+  a.limbs[idx]
+
 # ############################################################
 #
 #                    BigInt primitives
@@ -78,7 +81,7 @@ template addImpl[bits](result: CTBool[Limb], a: var BigInt[bits], b: BigInt[bits
   for i in static(0 ..< a.limbs.len):
     let new_a = a.limbs[i] + b.limbs[i] + Limb(result)
     result = new_a.isMsbSet()
-    a[i] = ctl.mux(new_a and highLimb, a)
+    a[i] = ctl.mux(new_a and HighLimb, a)
 
 func add*[bits](a: var BigInt[bits], b: BigInt[bits], ctl: CTBool[Limb]): CTBool[Limb] =
   ## Constant-time big integer in-place addition
@@ -96,7 +99,7 @@ template subImpl[bits](result: CTBool[Limb], a: var BigInt[bits], b: BigInt[bits
   for i in static(0 ..< a.limbs.len):
     let new_a = a.limbs[i] - b.limbs[i] - Limb(result)
     result = new_a.isMsbSet()
-    a[i] = ctl.mux(new_a and highLimb, a)
+    a[i] = ctl.mux(new_a and HighLimb, a)
 
 func sub*[bits](a: var BigInt[bits], b: BigInt[bits], ctl: CTBool[Limb]): CTBool[Limb] =
   ## Constant-time big integer in-place addition

constantine/private/word_types_internal.nim

@@ -13,6 +13,41 @@
 
 import ../word_types
 
+func asm_x86_64_extMul(hi, lo: var uint64, a, b: uint64) {.inline.}=
+  ## Extended precision multiplication uint64 * uint64 --> uint128
+
+  # TODO !!! - Replace by constant-time, portable, non-assembly version
+  #          -> use uint128? Compiler might add unwanted branches
+
+  # MUL r/m64
+  # Multiply RAX by r/m64
+  #
+  # Inputs:
+  #   - RAX
+  #   - r/m
+  # Outputs:
+  #   - High word in RDX
+  #   - Low word in RAX
+
+  asm """
+    mulq %[operand]
+    : "=d" (`*hi`), "=a" (`*lo`) // Don't forget to dereference the var hidden pointer
+    : "a" (`a`), [operand] "rm" (`b`)
+    : // no clobbered registers
+  """
+
+func unsafe_extendedPrecMul(hi, lo: var Ct[uint64], a, b: Ct[uint64]) {.inline.}=
+  ## Extended precision multiplication uint64 * uint64 --> uint128
+  ##
+  ## TODO, at the moment only x86_64 architecture are supported
+  ##       as we use assembly.
+  ##       Also we assume that the native integer division
+  ##       provided by the PU is constant-time
+
+  # Note, using C/Nim default `*` is inefficient
+  # and complicated to make constant-time
+  # See at the bottom.
+
 func asm_x86_64_div2n1n(q, r: var uint64, n_hi, n_lo, d: uint64) {.inline.}=
   ## Division uint128 by uint64
   ## Warning ⚠️ :
@@ -20,6 +55,7 @@ func asm_x86_64_div2n1n(q, r: var uint64, n_hi, n_lo, d: uint64) {.inline.}=
   ##   - if n_hi > d result is undefined
 
   # TODO !!! - Replace by constant-time, portable, non-assembly version
+  #          -> use uint128? Compiler might add unwanted branches
 
   # DIV r/m64
   # Divide RDX:RAX (n_hi:n_lo) by r/m64
@@ -27,7 +63,7 @@ func asm_x86_64_div2n1n(q, r: var uint64, n_hi, n_lo, d: uint64) {.inline.}=
   # Inputs
   #   - numerator high word in RDX,
   #   - numerator low word in RAX,
-  #   - divisor as rm parameter (register or memory at the compiler discretion)
+  #   - divisor as r/m parameter (register or memory at the compiler discretion)
   # Result
   #   - Quotient in RAX
   #   - Remainder in RDX
@@ -44,6 +80,10 @@ func unsafe_div2n1n*(q, r: var Ct[uint64], n_hi, n_lo, d: Ct[uint64]) {.inline.}
   ##   - if n_hi == d, quotient does not fit in an uint64
   ##   - if n_hi > d result is undefined
   ##
+  ## To avoid issues, n_hi, n_lo, d should be normalized.
+  ## i.e. shifted (== multiplied by the same power of 2)
+  ## so that the most significant bit in d is set.
+  ##
   ## TODO, at the moment only x86_64 architecture are supported
   ##       as we use assembly.
   ##       Also we assume that the native integer division
@@ -53,7 +93,7 @@ func unsafe_div2n1n*(q, r: var Ct[uint64], n_hi, n_lo, d: Ct[uint64]) {.inline.}
   # and complicated to make constant-time
   # See at the bottom.
   #
-  # Furthermore compilers try to substitute division
+  # Furthermore compilers may try to substitute division
   # with a fast path that may have branches. It might also
   # be the same at the hardware level.
 
@@ -65,7 +105,14 @@ func unsafe_div2n1n*(q, r: var Ct[uint64], n_hi, n_lo, d: Ct[uint64]) {.inline.}
     asm_x86_64_div2n1n(T(q), T(r), T(n_hi), T(n_lo), T(d))
 
 when isMainModule:
+  block: # Multiplication
+    var hi, lo: uint64
 
+    asm_x86_64_extMul(hi, lo, 1 shl 32, 1 shl 33) # 2^65
+    doAssert hi == 2
+    doAssert lo == 0
+
+  block: # Division
     var q, r: uint64
 
     # (1 shl 64) div 3
@@ -78,6 +125,49 @@ when isMainModule:
     doAssert q == 6148914691236517205'u64
     doAssert r == 1
 
+# ##############################################################
+#
+#  Non-constant-time portable extended precision multiplication
+#
+# ##############################################################
+
+# implementation from Stint: https://github.com/status-im/nim-stint/blob/edb1ade37309390cc641cee07ab62e5459d9ca44/stint/private/uint_mul.nim#L91-L135
+
+# template extPrecMulImpl(result: var UintImpl[uint64], op: untyped, u, v: uint64) =
+#   const
+#     p = 64 div 2
+#     base: uint64 = 1 shl p
+#
+#   var
+#     x0, x1, x2, x3: uint64
+#
+#   let
+#     ul = lo(u)
+#     uh = hi(u)
+#     vl = lo(v)
+#     vh = hi(v)
+#
+#   x0 = ul * vl
+#   x1 = ul * vh
+#   x2 = uh * vl
+#   x3 = uh * vh
+#
+#   x1 += hi(x0)          # This can't carry
+#   x1 += x2              # but this can
+#   if x1 < x2:           # if carry, add it to x3
+#     x3 += base
+#
+#   op(result.hi, x3 + hi(x1))
+#   op(result.lo, (x1 shl p) or lo(x0))
+#
+# func extPrecMul*(result: var UintImpl[uint64], u, v: uint64) =
+#   ## Extended precision multiplication
+#   extPrecMulImpl(result, `=`, u, v)
+#
+# func extPrecAddMul(result: var UintImpl[uint64], u, v: uint64) =
+#   ## Extended precision fused in-place addition & multiplication
+#   extPrecMulImpl(result, `+=`, u, v)
+
 # ############################################################
 #
 #             Non-constant-time portable div2n1n