Add SWAP17-SWAP24, DUP17 - DUP24, SWAPN, DUPN, and EXCHANGE
instructions. The arbitrary depth operations must be preceded by PUSH1
instructions defining operands.
Due to the nature of some compilers, deeper stack access is a desirable VM feature. Previous attempts either required code versioning, like the EVM Object Format (EOF), or caused the behavior of some deployed contracts to change, due to the interpretation of new immediates.
This is a pragmatic approach to introducing the desired functionality. It reuses instruction semantics that have been historically agreed on, instead of new, complex encodings and containers.
Let top - N be the Nth most recently pushed value on the stack, and top - 0 be the most recent.
If any of the following instructions reference a stack element beyond the current length of the stack, causing a stack underflow, abort with an exceptional halt.
SWAPXX and DUPXXAdd the following new instructions:
SWAP17, SWAP18, ..., SWAP24: 0xb0, 0xb1, ..., 0xb7.DUP17, DUP18, ..., DUP24: 0xb8, 0xba, ..., 0xbf.Let SWAPXX and DUPXX refer to the static instructions defined above. XX is
defined as the stack element they are referencing.
The operation SWAPXX swaps the top element with the top-XX element. The
operation DUPXX duplicates the top-XX element and pushes the copy to the top
of the stack.
SWAPN and DUPNAdd the following new instructions:
SWAPN: 0xc0.DUPN: 0xc1.Both operations take a single argument from the stack, N. This argument must
be provided by a PUSH1 operation immediately preceding the SWAPN and DUPN
instructions. Failure to follow this calling convention will result in an
out-of-gas error. If N is zero, fail with out-of-gas error.
SWAPN pops N from the stack and swaps the new top stack element with the
top-N stack element.
DUPN pops N from the stack and push on a copy of the top-N-1 stack element.
EXCHANGEAdd the following new instruction:
EXCHANGE: 0xc2.The EXCHANGE instruction takes a single argument from the stack X and
deconstructs it into two operands, N and M. N is X >> 4 and M is X & 0x0F. The argument X must be provided by a PUSH2 operation immediately
preceding the EXCHANGE instruction. Failure to follow this calling convention
will result in an out-of-gas error. If either N or M are zero, fail with
out-of-gas error.
EXCHANGE pops X from the stack and will swap the stack element at index
N-1 with the stack element at M-1.
All operations cost 3 gas. Preceding push operations are charged separately
according to the gas schedule.
SWAPs and DUPsThe main trade off between using the constant SWAPXX or DUPXX instructions
versus the dynamic SWAPN or DUPN instructions is that the dynamic
instructions require an additional two bytes in the form of a preceding PUSH1
operation, whereas the constant versions require no additional bytes.
EXCHANGESince SWAP1 and DUP1 operate on the top of the stack, it seems fitting that
EXCHANGE(1, 2) operate on the top and top-1.
No backward compatibility issues found.
TODO
When verifying the preceding PUSH operations, client implementers must ensure
that the preceding bytes are not part of a longer segment of push data (e.g.
0x6301026001b0 should error). This can be done efficiently by checking if
pc-2 is a valid jump destination. If it is, then the PUSH instruction will
have been executed as expected.
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