deal with and test zero-len case

evm/src/cpu/kernel/asm/bignum/add.asm

@@ -4,6 +4,11 @@
 // Adds two bignums of the same given length. Assumes that len > 0.
 // Replaces a with a + b, leaving b unchanged, and returns the final carry.
 global add_bignum:
+    // stack: len, a_start_loc, b_start_loc, retdest
+    DUP1
+    // stack: len, len, a_start_loc, b_start_loc, retdest
+    ISZERO
+    %jumpi(len_zero)
     // stack: len, a_start_loc, b_start_loc, retdest
     PUSH 0
     // stack: carry=0, i=len, a_cur_loc=a_start_loc, b_cur_loc=b_start_loc, retdest
@@ -55,3 +60,11 @@ add_end:
     SWAP1
     // stack: retdest, carry_new
     JUMP
+len_zero:
+    // stack: len, a_start_loc, b_start_loc, retdest
+    %pop3
+    // stack: retdest
+    PUSH 0
+    // stack: carry=0, retdest
+    SWAP1
+    JUMP

evm/src/cpu/kernel/asm/bignum/addmul.asm

@@ -4,6 +4,10 @@
 // Sets a[0:len] += b[0:len] * val, and returns the carry.
 global addmul_bignum:
     // stack: len, a_start_loc, b_start_loc, val, retdest
+    DUP1
+    // stack: len, len, a_start_loc, b_start_loc, val, retdest
+    ISZERO
+    %jumpi(len_zero)
     PUSH 0
     // stack: carry=0, i=len, a_cur_loc=a_start_loc, b_cur_loc=b_start_loc, val, retdest
 addmul_loop:
@@ -97,3 +101,11 @@ addmul_end:
     SWAP1
     // stack: retdest, carry_new
     JUMP
+len_zero:
+    // stack: len, a_start_loc, b_start_loc, val, retdest
+    %pop4
+    // stack: retdest
+    PUSH 0
+    // stack: carry=0, retdest
+    SWAP1
+    JUMP

evm/src/cpu/kernel/asm/bignum/mul.asm

@@ -7,6 +7,10 @@
 global mul_bignum:
     // stack: len, a_start_loc, b_start_loc, output_loc, retdest
     DUP1
+    // stack: len, len, a_start_loc, b_start_loc, output_loc, retdest
+    ISZERO
+    %jumpi(len_zero)
+    DUP1
     // stack: n=len, len, a_start_loc, bi=b_start_loc, output_cur=output_loc, retdest
 mul_loop:
     // stack: n, len, a_start_loc, bi, output_cur, retdest
@@ -51,3 +55,8 @@ mul_end:
     %pop5
     // stack: retdest
     JUMP
+len_zero:
+    // stack: len, a_start_loc, b_start_loc, output_loc, retdest
+    %pop4
+    // stack: retdest
+    JUMP

evm/src/cpu/kernel/asm/bignum/shr.asm

@@ -8,7 +8,7 @@ global shr_bignum:
     DUP1
     // stack: len, len, start_loc, retdest
     ISZERO
-    %jumpi(shr_end_len_zero)
+    %jumpi(len_zero)
     // stack: len, start_loc, retdest
     DUP2
     // stack: start_loc, len, start_loc, retdest
@@ -60,7 +60,7 @@ shr_end:
     %pop3
     // stack: retdest
     JUMP
-shr_end_len_zero:
+len_zero:
     // stack: len, start_loc, retdest
     %pop2
     // stack: retdest

evm/src/cpu/kernel/tests/bignum.rs

@@ -104,6 +104,15 @@ fn prepare_two_bignums_diff(bit_size: usize) -> (BigUint, BigUint, U256, Vec<U25
     (a, b, length, memory)
 }
 
+fn prepare_two_bignums_zero(_bit_size: usize) -> (BigUint, BigUint, U256, Vec<U256>) {
+    let a = BigUint::zero();
+    let b = BigUint::zero();
+    let length: U256 = bignum_len(&a).into();
+    let memory = pad_bignums(&[a.clone(), b.clone()], length.try_into().unwrap());
+
+    (a, b, length, memory)
+}
+
 fn test_shr_bignum<F>(prepare_bignum_fn: &F) -> Result<()>
 where
     F: Fn(usize) -> (BigUint, U256, Vec<U256>),
@@ -356,6 +365,7 @@ fn test_add_bignum_all() -> Result<()> {
     test_add_bignum(&prepare_two_bignums_max)?;
     test_add_bignum(&prepare_two_bignums_min)?;
     test_add_bignum(&prepare_two_bignums_diff)?;
+    test_add_bignum(&prepare_two_bignums_zero)?;
 
     Ok(())
 }
@@ -366,6 +376,7 @@ fn test_addmul_bignum_all() -> Result<()> {
     test_addmul_bignum(&prepare_two_bignums_max)?;
     test_addmul_bignum(&prepare_two_bignums_min)?;
     test_addmul_bignum(&prepare_two_bignums_diff)?;
+    test_addmul_bignum(&prepare_two_bignums_zero)?;
 
     Ok(())
 }
@@ -376,6 +387,7 @@ fn test_mul_bignum_all() -> Result<()> {
     test_mul_bignum(&prepare_two_bignums_max)?;
     test_mul_bignum(&prepare_two_bignums_min)?;
     test_mul_bignum(&prepare_two_bignums_diff)?;
+    test_mul_bignum(&prepare_two_bignums_zero)?;
 
     Ok(())
 }