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Comment by otabdeveloper4 | Hacker News Reader

gcc is better than clang on every metric. Sorry, it's the facts. ¯\_(ツ)_/¯

Whoa, really? Since when??

After a lifetime of using GCC, I (like many others) moved to clang a few years ago, out of frustration with the slow development of GCC, a desire to use new C++ features, and stayed because of the superior error messages and, in my use cases anyway, superior code generation.

In addition, I gather it's a much cleaner and easier to maintain code base. As a result, we get to have cool things like emscripten, llvmpipe, all sorts of static analysis tools that would be more challenging to build in the GCC universe, and much more.

Honestly, I thought GCC was slowing down development wise.

Is GCC worth trying again? Can you name a few "cool new things" I can do with GCC that I can't with clang? There's plenty of the opposite...

LLVM always delivers the cooler stuff I'll admit but GCC is ahead as an optimizing compiler, general lang support and generating cryptic messages.

Interesting, I've generally found that clang is better at optimising, especially when it comes to auto-vectorisation.

Agreed, autovec is in my experience where clang really shines.

Gcc pretty consistently delivers slightly faster code.

Not in my experience! Though it was a few years ago, I'll have to re-evaluate. In the past, when doing numeric/scientific programming -- lots of dataflow, it was much easier to "coax" clang to generate the right (performant) assembly than GCC. I guess my use is less typical -- the internet agrees with you. :)

Hurrah for competition!

(of course, neither GCC or clang could _ever_ beat ICC at some of my tests.. grumble)

"(of course, neither GCC or clang could _ever_ beat ICC at some of my tests.. grumble)"

They never tried, honestly - ICC had plenty of defaults that were targeted at performance above correctness.

That's fine - but it wasn't what either clang or gcc was going to go for. Even when trying to compare apples to apples, folks often compared them by trying to get GCC/Clang to emulate the correctness level of ICC (IE give GCC/Clang more freedom), rather than the other way around :)

Which, again, similarly understandable, but also a thing that you'd have to spend a while on in GCC/Clang to get to a reasonable place)

Small-number-of-target compilers like ICC are also fundamentally easier. Lots of techniques (IE optimal register allocation[1], etc) that add up to performance gains are a lot more tractable to really good applied engineering when they don't have to be so general.

Similarly small-target-market compilers are also easier. Over time, high performance was literally ICC's only remaining market. So they can spend their days working on that. GCC/Clang had to care about a lot more.

[1] This happens to now be a bad example because advances finally made this particular thing tractable. But it took years more, and feel free to replace it with something like "optimal integrated register allocation and scheduling" or whatever is still intractable to generalize.

You sound like you know what you're talking about.

> They never tried, honestly - ICC had plenty of defaults that were targeted at performance above correctness.

Is it possible to get ICC level performance out of open source tools? Much of my CPU bound work relates to array signal processing, which, if you can code it right :) lends itself heavily to SIMD branchless pipelines. Plus some scatter gathers on a group of other cores to calculate a sparse crosscorrelation tensor.

I would love to be able to get same or better performing code out of clang or gcc, even it it takes 2-4x more work than with ICC...

This is often true for SPEC, but not true for real world code.

Or at least, we extensively tested this at Google, before, during, and after the move to LLVM.

On many thousands of libraries, binaries, etc, made up of hundreds of millions of lines of C++.

While there were wins and losses, on average, LLVM was a consistent net positive.

That was true despite having spent 5+ years of having a large team dedicated to doing nothing but finding places to improve performance of GCC compiled code (and contributing back patches), and doing so very successfully.

That said, as time approaches infinity, the compilers are going to generate the best code for the things that someone took the time to analyze and make the compiler better at.

There is, in the end, no magic that makes GCC better than LLVM or vice versa. The vast majority of it is tuning and improving things little by little for whatever targets someone is trying to improve.

I can only go by the results I measure. There certainly are cases where Clang does better than Gcc, but the difference is anyway rarely more than a couple of percent. Occasionally one finds a factor of two one way or the other, e.g. where one compiler has noticed it can use CMOV without penalty.

proof?

Keyword there is "slightly".

C++20 support comes to mind, the plethora of supported hardware as well.

> C++20 support?

Not that I think it matters but this is, of everything, the strongest argument to me. For shame, clang, for shame! I switched because it had better modern c++ support. Guess the winds are changing.

GCC is quite up to date. Also, the code generation used to be better. I do not know right now. But I think it still might have a slight advantage.

I've always preferred clang for its better lints, and friendlier error messages (sans some decrepit parts around templates that are equally horrendous everywhere)...

And theoretically clang has better ASM output in some cases I say theoretically, because it's been shown that GCC's "worse" ASM performs better; I'm not really an architecture aficionad, so I can't comment as to why that is.

Also, it's been a few years now since I did C/C++. So, maybe these are no longer the case.

Anyway, I've kinda pointed out what I like about clang over gcc, but I'd be curious what you prefer in gcc.

I wonder how up to date is the old belief of better error messages. The was some great redhat blogs about structured error messages in newer GCC.

GCC has gotten better indeed, but it's still in a different league than Clang. I still get into situations where I can't make heads-or-tails of what GCC is saying to me, which can usually be easily solved by switching to Clang.

One problem here is that GCC emits certain warnings as part of the optimizer, which results in many false positives that are essentially impossible for the lay programmer to understand. For example, jump threading might duplicate some code path and propagate constants in it, and then warn about an uninitialized variable / out of range access / etc in that code-path, even though it does not exist in that form in the original program.

Funny enough, this is exactly the kind of situation where other people cry "why did the compiler not warn me about basing optimizations on UB".

I don't do C++ anymore but I will forever remember the Vtable hell "messages" when doing OOP and doing a slight unintuitive mistake about destructor or constructor. Is this still a thing in clang?

Ok, that's kinda true I suppose; I remember that GCC's error messages have indeed gotten better after a major version release.

Also, when you say structured, do you mean errors over LSP, or do you mean more structure in the formatting when reporting in CLI?

GCC returns pretty nice, colored and structured error messages in CLI for quite some time.

I meant in cli a bit like rust arrows logs

clang doesn't output asm

I'm not sure what you mean? You can use `-S`

What is the status of ubsan, msan, tsan and others support for GCC though? Last time I checked they were a bit behind. I agree GCC make clang obscolete regarding C++ support and even performance. I don't know a comprehensive alternative to clang linter but I'm sure there are a few.

Given that llvm receive more human resources than gcc by far, I once expected it to outperform gcc generally (e.g support for polyhedral optimizations, BOLT, etc) Unfortunately weirdly it seems llvm performance is mostly stagnant. I personally suspect we are reaching increasingly diminishing returns with AOT and that the performance graal would be a hybrid that also does JIT at runtime (beyond PGO therefore) and more interpretable than BOLT

At work GCC is a lot slower than clang, the project uses a bit much template magic but still, clang is faster.

Where does this GCC is faster thing come from? I personally havent experienced it

I mean faster runtime performance, I have no clue about compilation time. Well I'm basing this on the countless benchmarks I've seen, e.g. on phoronix over the decade. Also you have to understand that Clang -O2 is (was) "unfair" as GCC did not enable autovectorization until -O3. This has (is being?) changed.

Old hype thing created by gcc fanboys :)

They always claims so.

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