solc compiles address(this).balance to SELFBALANCE since 0.5.13 — inline assembly selfbalance() saves nothing

Misconception worth correcting during gas-optimisation work: address(this).balance is not compiled as a generic BALANCE lookup. solc has emitted the SELFBALANCE opcode for it since 0.5.13, and since 0.5.14 the default target EVM version is Istanbul, so the optimisation is on by default in every modern 0.8.x build. Replacing it with assembly { b := selfbalance() } produces identical bytecode and only costs readability.

Why it is always safe: EIP-1884 defines SELFBALANCE as pushing the balance of the currently executing address, which is exactly the semantics of address(this).balance in every context — including inside a delegatecall, where address(this) is the proxy and SELFBALANCE likewise reports the executing (proxy) account. There is no case where the two differ, which is why the compiler can substitute unconditionally.

Cost: SELFBALANCE is priced at 5 gas (GasFastStep) rather than the 2 of truly constant pushes, because balance can change during execution and the value still needs a trie lookup; BALANCE(address) by contrast costs an account access (100 warm / 2600 cold post-Berlin). So the saving over someAddress.balance is large — but you already have it for free on this.

The one thing to check: the opcode requires the compiler's evmVersion to be istanbul or later, and the chain to support it. If you compile with an older evmVersion target (or deploy to a chain stuck pre-Istanbul), solc falls back to BALANCE and hand-written selfbalance() in inline assembly reverts with an invalid opcode. Set evmVersion deliberately in foundry.toml/hardhat config rather than relying on defaults when targeting exotic chains.

Source: adapted from a Stack Exchange answer by eth (question by Tilak Madichetti), https://ethereum.stackexchange.com/questions/164525/what-keeps-solidity-compiler-from-optimizing-addressthis-balance-to-selfbal — CC BY-SA 4.0