EVM JSON outputs are generated even when not requested

[DaniPopes]

Description

Certain evm.*.* output is generated in memory even when not requested through outputSelection. This wastes resources on computing values that are discarded immediately after.

In Foundry, we updated the outputSelection to only include specific fields of evm.bytecode.* as we don't make use of all of them, which was a substantial win in compilation times: foundry-rs/compilers#231. The biggest contributor to this was excluding generatedSources from the JSON, as these can become very large.

The default output selection is now:

["abi","evm.bytecode.object","evm.bytecode.sourceMap","evm.bytecode.linkReferences","evm.deployedBytecode.object","evm.deployedBytecode.sourceMap","evm.deployedBytecode.linkReferences","evm.deployedBytecode.immutableReferences","evm.methodIdentifiers","metadata"]

However, I profiled the compiler and noticed that it was still calling generatedSources.

Here's a profile generated from forge build --use <solc 0.8.29-develop.2024.10.22+commit.9c4995a9.mod.Linux.g++> on superform-xyz/superform-core @ 7b69077 (this is an older build of solc but it should still apply): https://share.firefox.dev/3BaYnLD, using samply

You can see that there is a 4% frame of CompilerStack::generatedSources.

I believe this happens in StandardCompiler. It is generated and passed eagerly to collectEVMObject when any evm.bytecode* output is requested

solidity/libsolidity/interface/StandardCompiler.cpp

Lines 1509 to 1522 in 8da621c

	evmData["bytecode"] = collectEVMObject(
	_inputsAndSettings.evmVersion,
	compilerStack.object(contractName),
	compilerStack.sourceMapping(contractName),
	compilerStack.generatedSources(contractName),
	false,
	[&](std::string const& _element) { return isArtifactRequested(
	_inputsAndSettings.outputSelection,
	file,
	name,
	"evm.bytecode." + _element,
	wildcardMatchesExperimental
	); }
	);

but then discarded if the individual evm.bytecode.generatedSources is not requested

solidity/libsolidity/interface/StandardCompiler.cpp

Lines 399 to 400 in 8da621c

	if (_artifactRequested("generatedSources"))
	output["generatedSources"] = std::move(_generatedSources);

The same applies to other fields, like sourceMap.

Environment

• Compiler version: 0.8.28, main
• Target EVM version (as per compiler settings): Paris
• Framework/IDE (e.g. Truffle or Remix): Foundry
• EVM execution environment / backend / blockchain client: Foundry
• Operating system: Linux

Steps to Reproduce

Compile anything with evm.bytecode.object output selection, see above.

[cameel]

Good find! #15460 solved part of the problem at CompilerStack level but I did not notice that StandardCompiler was also eagerly requesting it rather than doing it only when the output is there. Fixing this should shave off a bit more from the compilation time.