Clang vs. other open-source compilers

https://clang.llvm.org/comparison.html

111 51 6 years ago

clang.llvm.org

Excerpt:

"Clang can serialize its AST out to disk and read it back into another program, which is useful for whole program analysis.

GCC does not have this. GCC's PCH mechanism (which is just a dump of the compiler memory image) is related, but is architecturally only able to read the dump back into the exact same executable as the one that produced it (it is not a structured format)."

Clang, you had me at 'hello'.

The point I outlined above is just icing on the cake!

mhh__• 6 years ago

GCC can dump it's AST https://stackoverflow.com/questions/15800230/how-can-i-dump-...

Ace17• 6 years ago

GCC can dump its internal representation at almost every stage of the compilation process.

However, it's intended for diagnosing issues in the compilers or plugins ; it's definitely not meant to be used as an interoperable format to be loaded back into a program.

gcc doesn't even provide a way to specify the output path for the dump file (too bad, as reliable AST dumping could enable implementing ast-based-ccache (instead of preprocessed-code-based-ccache, for compilation-caching of preprocessor-less languages)).

SeekingMeaning• 6 years ago

I mean Elsa and and PCC are cool, but who actually uses them? Clang and GCC are pretty much all there is, but regardless it’s nice to know what people have tried/are trying to do. These bullet points in particular make me want to tinker with PCC and see how much it is (and is not) capable of:

The PCC source base is very small and builds quickly with just a C compiler.
PCC doesn't support Objective-C or C++ and doesn't aim to support C++.

vyjcgtyjb• 6 years ago

Many use Microsoft's and Intel's.

thenewnewguy• 6 years ago

This is a page about _open source_ compilers.

kick3• 6 years ago

PCC was at one time basically the standard C compiler, and you can port it to just about any architecture you want in less than a day.

It also shipped with many historic operating systems, Plan 9 from Bell Laboratories being one of them, if I remember correctly.

Taniwha• 6 years ago

And Unix

4ad2• 6 years ago

Pcc was never used in Plan 9. The Plan 9 C compilers[1][2] were written by Ken Thompson, and later reused in the Go toolchain.

[1] http://doc.cat-v.org/plan_9/4th_edition/papers/compiler

[2] http://doc.cat-v.org/plan_9/4th_edition/papers/comp

derjames• 6 years ago

I do not use Elsa nor PCC. However I use TCC extensively for both Windows and Linux in production code.

dman3• 6 years ago

Could you please tell why tcc works for your usecases? Asking so that I know when to reach for it.

gok2• 6 years ago

This is staggeringly out of date (it's from around 2007) and really ought to be taken down.

mattst88• 6 years ago

Could you provide a few examples of what information is out of date?

srott• 6 years ago

This one is also outdated:

https://www.phoronix.com/scan.php?page=article&item=gcc7-cla...

haven't checked recently, because gcc sped up and still produces better binaries

It can dump AST and much more: https://godbolt.org/

gok2• 6 years ago

GCC's diagnostic system has been completely re-written, for example

NilsIRL• 6 years ago

Link time optimization is possible with GCC.

notaplumber2• 6 years ago

GCC is licensed under the GPL license. Clang uses a BSD license ...

Looks like this hasn't been updated in a while. As of Clang 9.0 they migrated everything to the Apache 2.0 license, which is not nearly as permissive as BSD. Apache 2.0 mixes US Contract law with Copyright law, and that is considered wholly "not permissive enough" by many, most notably OpenBSD which is stuck on Clang 8.0.1. They also migrated the libc++/libc++abi C++ standard libraries from MIT to Apache 2.0 as well (which was a real dick move), but they don't care.

Joky• 6 years ago

Minor precision: it is Apache 2.0 with LLVM exceptions, the exceptions are intended to address libc++ and other runtime use-cases I believe.

chipperyman573• 6 years ago

Why were the license changes made?

pnako• 6 years ago

https://llvm.org/foundation/relicensing/

Addressing the patents question seems to be the main reason.

mhd• 6 years ago

Interesting, I didn't know that And that after the BSDs were quite happy that a viable non-GPL compiler arrived on the market. Wasn't their final switchover just in 2017?

trasz2• 6 years ago

BSDs in general are quite happy, since Apache is pretty much just BSD with patents thing sorted out. The only unhappy ones are OpenBSD folks.

eeZah7Ux2• 6 years ago

Apache 2.0 mixes US Contract law with Copyright law, and that is considered wholly "not permissive enough" by many

And also too permissive by many other. It does not protect from tivoization, freeloading and yet it's not compatible with GPLv2.

eeZah7Ux2• 6 years ago

The silent downvotes are telling.

nwallin2• 6 years ago

I mean this is fine for an advertisement, but it woefully oversells clang if you're trying to meaningfully compare these things. A lot of what they're saying is misleading and or false.

In the early days, clang was significantly faster in compilation than GCC. They also barely implemented any code optimization. Now that clang generates code which is about 90% as fast, generally, as C++, its compilation speeds and memory usage have understandably bloated considerably.

Note that I say 90% as fast generally. It still hasn't caught up completely.

Clang pioneered LTO, but GCC does it better now.

Other people have mentioned gcc's previously terrible error message and inability to dumb ASTs.

I don't think this is up to HN standards.

dman3• 6 years ago

gcc error messages have improved massively since llvm came out.

nwallin2• 6 years ago

Yes.

I don't believe clang is any better than GCC anymore. But other commenters have talked about it, and I don't want to be redundant.

dxf• 6 years ago

I think it's great that gcc, Clang, and MSVC all push each other to improve their compile/link performance, code generation, error messaging, etc.

mehrdadn• 6 years ago

Clang pioneered LTO, but GCC does it better now.

Sorry, what? VC++ already had link-time code generation by 2005. (No clue exactly which year it was introduced though.)

marklacey• 6 years ago

It was first released in VC++ in 2002.

I don’t recall if HP actually shipped it, but I know they had a tech report or research paper around 1994 that did LTO.

I have a vague recollection that DEC may have shipped it in the 90s as well.

steve1977• 6 years ago

The headline is clang vs. other open-source compilers, not sure if VC++ qualifies.

bla3• 6 years ago

As far as I know that page is many years old, from when clang was very new and comparing to e.g. Elsa made sense. The gcc comparison is probably pretty out of date.

qeqeqeqe• 6 years ago

This info is from 2007, the title should reflect that. This is not a useful post.

BinaryIdiot• 6 years ago

As far as I can tell this page first appeared in 2008. Many of these points are outdated (some woefully so).

This shouldn't be on the frontpage of HN. If anything, Clang should take this down or revise it.

fouc• 6 years ago

Clang takes up to 90GB of disk space [0]

[0] https://lists.llvm.org/pipermail/llvm-dev/2019-April/132028....

endorphone• 6 years ago

The overwhelming bulk of users are going to install a binary package, not build from source.

LLVM 9.0 with Clang, a huge array of tools, etc, is 1.9GB on macOS.

swiley• 6 years ago

The point of open source is that individuals from the community can read and modify their tools. Anything that makes that harder is a bad thing, it might be justified but it’s still bad.

justinclift• 6 years ago

Note that's only when building in debug mode, which doesn't seem to be needed by many people.

Most developers who compile from source (eg working on LLVM itself) don't need that.

Building in release mode is much smaller probably ~5GB or so (from rough memory, it's been a while).

Clang vs. other open-source compilers

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