From 586c71b3472b3b0064e61fd0ff157998a7cf3262 Mon Sep 17 00:00:00 2001
From: Mamy Ratsimbazafy <mamy_github@numforge.co>
Date: Wed, 6 Sep 2023 20:21:42 +0200
Subject: [PATCH] modexp: accel exponent = 1

---
 benchmarks/bench_evm_modexp_dos.nim           | 62 ++++++++++++++++++-
 .../arithmetic/bigints_views.nim              |  6 ++
 .../arithmetic/limbs_mod2k.nim                |  7 +++
 3 files changed, 72 insertions(+), 3 deletions(-)

diff --git a/benchmarks/bench_evm_modexp_dos.nim b/benchmarks/bench_evm_modexp_dos.nim
index 6f18c221..312e36c6 100644
--- a/benchmarks/bench_evm_modexp_dos.nim
+++ b/benchmarks/bench_evm_modexp_dos.nim
@@ -10,9 +10,9 @@ proc report(op: string, elapsedNs: int64, elapsedCycles: int64, iters: int) =
   let cycles = elapsedCycles div iters
   let throughput = 1e9 / float64(ns)
   when SupportsGetTicks:
-    echo &"{op:<45} {throughput:>15.3f} ops/s {ns:>16} ns/op {cycles:>12} CPU cycles (approx)"
+    echo &"{op:<70} {throughput:>15.3f} ops/s {ns:>16} ns/op {cycles:>12} CPU cycles (approx)"
   else:
-    echo &"{op:<45} {throughput:>15.3f} ops/s {ns:>16} ns/op"
+    echo &"{op:<70} {throughput:>15.3f} ops/s {ns:>16} ns/op"
 
 template bench(fnCall: untyped, ticks, ns: var int64): untyped =
   block:
@@ -366,6 +366,60 @@ proc dos2c() =
   report("EVM Modexp - 1,1,121 - exponent=7 and odd modulus", nanoseconds, ticks, execsEIP2565)
   echo "Total time: ", nanoseconds.float64 / 1e6, " ms for ", execsEIP2565, " iterations"
 
+proc dos2d() =
+  # odd variation with no shortcut and power of 2 modulus
+
+  let input = [
+    # Length of base (1)
+    uint8 0x00,
+          0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01,
+
+    # Length of exponent (1)
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01,
+
+    # Length of modulus (121)
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x79,
+
+    # Base
+    0x33,
+
+    # Exponent
+    0x07,
+
+    # Modulus
+    0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+  ]
+
+  var r = newSeq[byte](121)
+  var ticks, nanoseconds: int64
+
+  let (gasFeeEIP198, gasFeeEIP2565) = computeGasFee(input)
+  const blockSize = 30000000
+
+  let execsEIP198 = blockSize div gasFeeEIP198
+  let execsEIP2565 = blockSize div gasFeeEIP2565
+
+  echo "Gas cost: ", gasFeeEIP198, " gas (EIP-198) - ", execsEIP198, " executions per block"
+  echo "Gas cost: ", gasFeeEIP2565, " gas (EIP-2565) - ", execsEIP2565, " executions per block"
+
+  for i in 0 ..< execsEIP2565:
+    bench(
+      (let _ = r.eth_evm_modexp(input)),
+      ticks, nanoseconds)
+
+  report("EVM Modexp - 1,1,121 - exponent=7 and power-of-2 modulus", nanoseconds, ticks, execsEIP2565)
+  echo "Total time: ", nanoseconds.float64 / 1e6, " ms for ", execsEIP2565, " iterations"
+
 dos1()
 echo "\n"
 dos2()
@@ -374,4 +428,6 @@ dos2a()
 echo "\n"
 dos2b()
 echo "\n"
-dos2c()
\ No newline at end of file
+dos2c()
+echo "\n"
+dos2d()
\ No newline at end of file
diff --git a/constantine/math_arbitrary_precision/arithmetic/bigints_views.nim b/constantine/math_arbitrary_precision/arithmetic/bigints_views.nim
index b95d4405..2c0f9384 100644
--- a/constantine/math_arbitrary_precision/arithmetic/bigints_views.nim
+++ b/constantine/math_arbitrary_precision/arithmetic/bigints_views.nim
@@ -57,6 +57,12 @@ func powOddMod_vartime*(
 
   let aBits  = a.getBits_LE_vartime()
   let mBits  = M.getBits_LE_vartime()
+  let eBits  = exponent.getBits_BE_vartime()
+
+  if eBits == 1:
+    r.view().reduce(a.view(), aBits, M.view(), mBits)
+    return
+
   let L      = wordsRequired(mBits)
   let m0ninv = M[0].negInvModWord()
   var rMont  = allocStackArray(SecretWord, L)
diff --git a/constantine/math_arbitrary_precision/arithmetic/limbs_mod2k.nim b/constantine/math_arbitrary_precision/arithmetic/limbs_mod2k.nim
index 2d50379f..ac33f80b 100644
--- a/constantine/math_arbitrary_precision/arithmetic/limbs_mod2k.nim
+++ b/constantine/math_arbitrary_precision/arithmetic/limbs_mod2k.nim
@@ -115,6 +115,13 @@ func powMod2k_vartime*(
     r[0] = One  # x⁰ = 1, even for 0⁰
     return
 
+  if msb == 0: # exponent is 1
+    for i in 0 ..< min(r.len, a.len):
+      # range [r.len, a.len) will be truncated (mod 2ᵏ)
+      r[i] = a[i]
+    r.mod2k_vartime(k)
+    return
+
   if a.isEven().bool:
     let aTrailingZeroes = block:
       var i = 0