Default recursive template instantiations is lower than recommended

[llvmbot]

Bugzilla Link	18417
Version	trunk
OS	Linux
Attachments	Patch to increase -ftemplate-depth default value to 1024
Reporter	LLVM Bugzilla Contributor
CC	@zygoloid

Extended Description

N3690 annex B states:

• Recursively nested template instantiations, including substitution during template argument deduction (14.8.2) [1024].

Though this is not a compliance issue, IMO, it would be nice if clang could do -ftemplate-depth=1024 by default, which is the case with gcc (gcc-4.8 at least).

$ man gcc
[snipped...]
       -ftemplate-depth=n
           Set the maximum instantiation depth for template classes to n.  A limit on the template instantiation depth
           is needed to detect endless recursions during template class instantiation.  ANSI/ISO C++ conforming
           programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++0x).
[snipped...]

[zygoloid]

On some systems, increasing the recursion depth leads to stack overflows; the current limit was determined through experimentation, and we can't increase it without other code changes to reduce stack usage or to increase the stack rlimit.

[llvmbot]

On some systems, increasing the recursion depth leads to stack overflows

Which systems are you referring to?

we can't increase it without other code changes to reduce stack usage or to increase the stack rlimit

If the rlimit route is acceptable, I would like to look into it and hopefully come out with a solution proposal.

I am interested in this issue mainly for two reasons:

1. I am in the process of migrating company projects from gcc to clang, and one of our projects, an llvm front end, needs -ftemplate-depth=1024 (512 doesn't do the trick) to compile its spirit-based parser
2. I like clang/llvm, I am interested in helping making clang a better and more conforming compiler. This issue seems to be a good place to start for someone without significant prior knowledge in clang's internals.

[zygoloid]

On some systems, increasing the recursion depth leads to stack overflows

Which systems are you referring to?

I don't remember exactly, but I could probably do some archaeology to find out. I vaguely recall the current limit came from some flavor of BSD.

we can't increase it without other code changes to reduce stack usage or to increase the stack rlimit

If the rlimit route is acceptable, I would like to look into it and
hopefully come out with a solution proposal.

I think this is probably a viable approach (on systems that support it), but there may be problems here that I'm not aware of. I'd suggest you mail cfe-dev before you start working on this, in case someone has a strong case against doing this.

one of our projects needs -ftemplate-depth=1024 to compile its spirit-based parser

Ouch =)

[llvmbot]

Email to cfe-dev sent http://lists.cs.uiuc.edu/pipermail/cfe-dev/2014-January/034529.html

Thanks.

[llvmbot]

I would also like to see the default template instantiation depth raised. 256 is low and hurts the gcc->clang migration story. Were any experiments done with rlimit?

[zygoloid]

I landed changes to increase clang's rlimit automatically yesterday. We go up to 8MiB (whereas it seems GCC asks for 64MiB); in my limited testing, this is enough to reach a template depth of 1024, at least in common cases.

I'll try increasing this and see if any buildbots break.

[zygoloid]

Increased in r278983 and r278984. It remains to be seen if this makes buildbots angry on some platform.

[llvmbot]

rockstar That's the kind of maintainer I like my open source projects to have. One that can look into the future and see that I'm about to agitate on a bug report and preemptively fix it. :-)

[llvmbot]

mentioned in issue llvm/llvm-bugzilla-archive#38090

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (llvmbot)

| | | | --- | --- | | Bugzilla Link | [18417](https://llvm.org/bz18417) | | Version | trunk | | OS | Linux | | Attachments | [Patch to increase -ftemplate-depth default value to 1024](https://user-images.githubusercontent.com/60944935/143748768-7b0d9749-fc22-48b6-95ff-2a0a601e2766.gz) | | Reporter | LLVM Bugzilla Contributor | | CC | @zygoloid |

Extended Description

N3690 annex B states:

• Recursively nested template instantiations, including substitution during template argument deduction (14.8.2) [1024].

Though this is not a compliance issue, IMO, it would be nice if clang could do -ftemplate-depth=1024 by default, which is the case with gcc (gcc-4.8 at least).

$ man gcc
[snipped...]
       -ftemplate-depth=n
           Set the maximum instantiation depth for template classes to n.  A limit on the template instantiation depth
           is needed to detect endless recursions during template class instantiation.  ANSI/ISO C++ conforming
           programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++0x).
[snipped...]