Platform probing can be replaced with `dmd -Xi=compilerInfo -Xf=-` for newer compilers

[wilzbach]

Flow:

dub/source/dub/commandline.d

Lines 1012 to 1032 in cea2f15

	override int execute(Dub dub, string[] free_args, string[] app_args)
	{
	enforceUsage(
	!(m_importPaths && m_stringImportPaths),
	"--import-paths and --string-import-paths may not be used together."
	);
	enforceUsage(
	!(m_data && (m_importPaths || m_stringImportPaths)),
	"--data may not be used together with --import-paths or --string-import-paths."
	);
	// disable all log output to stdout and use "writeln" to output the JSON description
	auto ll = getLogLevel();
	setLogLevel(max(ll, LogLevel.warn));
	scope (exit) setLogLevel(ll);
	string package_name;
	enforceUsage(free_args.length <= 1, "Expected one or zero arguments.");
	if (free_args.length >= 1) package_name = free_args[0];
	setupPackage(dub, package_name);

dub/source/dub/commandline.d

Lines 618 to 622 in cea2f15

	protected void setupPackage(Dub dub, string package_name, string default_build_type = "debug")
	{
	if (!m_compilerName.length) m_compilerName = dub.defaultCompiler;
	m_compiler = getCompiler(m_compilerName);
	m_buildPlatform = m_compiler.determinePlatform(m_buildSettings, m_compilerName, m_arch);

dub/source/dub/compilers/dmd.d

Lines 56 to 68 in cea2f15

	BuildPlatform determinePlatform(ref BuildSettings settings, string compiler_binary, string arch_override)
	{
	string[] arch_flags;
	switch (arch_override) {
	default: throw new Exception("Unsupported architecture: "~arch_override);
	case "": break;
	case "x86": arch_flags = ["-m32"]; break;
	case "x86_64": arch_flags = ["-m64"]; break;
	case "x86_mscoff": arch_flags = ["-m32mscoff"]; break;
	}
	settings.addDFlags(arch_flags);
	return probePlatform(compiler_binary, arch_flags ~ ["-quiet", "-c", "-o-"], arch_override);

dub/source/dub/compilers/compiler.d

Lines 119 to 124 in cea2f15

	protected final BuildPlatform probePlatform(string compiler_binary, string[] args, string arch_override)
	{
	import std.string : format;
	import dub.compilers.utils : generatePlatformProbeFile, readPlatformProbe;
	auto fil = generatePlatformProbeFile();

dub/source/dub/compilers/utils.d

Lines 245 to 252 in cea2f15

	NativePath generatePlatformProbeFile()
	{
	import dub.internal.vibecompat.core.file;
	import dub.internal.vibecompat.data.json;
	import dub.internal.utils;
	auto path = getTempFile("dub_platform_probe", ".d");

Probably it could be something simple as the timestamp of the compiler binary.
How about ~/.dub/compilers/<dmd|ldc|gdc>_timestamp?

Related PRs:

• Avoid creation of a temporary probe file for newer compilers #1316 (avoids the creation of this temporary file)
• Implement -probe for showing supported features dmd#7521 (implements -probe directly in the compiler)

[MartinNowak]

It took ~5ms last time I optimized it, hardly worth caching. Of course in the mean-time it was changed to import phobos again.

[MartinNowak]

#1318

[wilzbach]

It took ~5ms last time I optimized it, hardly worth caching.

But isn't this probing done on every DUB invocation?

[FeepingCreature]

It took ~5ms last time I optimized it, hardly worth caching.

edit: Nevermind, it's dub in general, not the probe invocation. Misread the strace, carry on.

[wilzbach]

FWIW dmd now supports dmd -Xi=compilerInfo -Xf=- since 2.079 which allows to avoid the entire probing.

[wilzbach]

@FeepingCreature note that the officially released dub binary still isn't build with LDC. You might want to build it with LDC locally for a huge speedup for "free"

[wilzbach]

Example: https://run.dlang.io/is/DiSyCl

[FeepingCreature]

Good idea, thank you!

[andre2007]

Can the probe result be stored in a cache file? On linux calling another executable is cheap but on windows with a OnAccess virus scanner this makes the single package file scenario slower.

[wilzbach]

Can the probe result be stored in a cache file?

I don't see any reason why it couldn't. I guess even just the timestamp and size of the compiler binary should be good enough for most (all?) probe caching purposes.