BZ 5012: Add FAQ entry for compiling with optimization

BZ 5012: Add FAQ entry for compiling with optimization

[Andreas Jaeger]

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The following adds an entry to the FAQ to mention why optimization is
needed for compiling glibc.  Ok to commit?

Andreas

2007-10-27  Andreas Jaeger  <aj@suse.de>,
	    Carlos O'Donell <carlos@systemhalted.org>

	[BZ #5012]
	* FAQ.in: Describe why glibc needs to be compiled with
	optimization.

============================================================
Index: FAQ.in
--- FAQ.in	8 Apr 2005 20:59:50 -0000	1.137
+++ FAQ.in	27 Oct 2007 15:14:50 -0000
@@ -387,6 +387,32 @@ yourself. Please remember that for each 
 patches required to get glibc HEAD into a runnable state. The best course
 of action is to determine if you have all the required patches.
 
+??	Why do I get:
+	`#error "glibc cannot be compiled without optimization"',
+	when trying to compile GNU libc with GNU CC?
+
+{AJ,CO} There are a couple of reasons why the GNU C library will not work
+correctly if it is not complied with optimzation.
+
+In the early startup of the dynamic loader (_dl_start), before
+relocation of the PLT, you cannot make function calls. You must inline
+the functions you will use during early startup, or call compiler
+builtins (__builtin_*).
+
+Without optimizations enabled GNU CC will not inline functions. The
+early startup of the dynamic loader will make function calls via an
+unrelocated PLT and crash. 
+
+Without auditing the dynamic linker code it would be difficult to remove
+this requirement. 
+
+Another reason is that nested functions must be inlined in many cases to
+avoid executable stacks.
+
+In practice there is no reason to compile without optimizations, therefore
+we require that GNU libc be compiled with optimizations enabled.
+
+
 ? Installation and configuration issues
 
 ??	Can I replace the libc on my Linux system with GNU libc?
@@ -1685,6 +1711,7 @@ Answers were given by:
 {AO} Alexandre Oliva, <aoliva@redhat.com>
 {BH} Bruno Haible, <haible@clisp.cons.org>
 {SM} Steven Munroe, <sjmunroe@us.ibm.com>
+{CO} Carlos O'Donell, <carlos@systemhalted.org>
 \f
 Local Variables:
  mode:outline

-- 
 Andreas Jaeger, Director Platform / openSUSE, aj@suse.de
  SUSE LINUX Products GmbH, GF: Markus Rex, HRB 16746 (AG Nürnberg)
   Maxfeldstr. 5, 90409 Nürnberg, Germany
    GPG fingerprint = 93A3 365E CE47 B889 DF7F  FED1 389A 563C C272 A126

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Re: BZ 5012: Add FAQ entry for compiling with optimization

[Andreas Jaeger]

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Andreas Jaeger <aj@suse.de> writes:

> The following adds an entry to the FAQ to mention why optimization is
> needed for compiling glibc.  Ok to commit?

Is it ok to commit this one?

Andreas

> Andreas
>
> 2007-10-27  Andreas Jaeger  <aj@suse.de>,
> 	    Carlos O'Donell <carlos@systemhalted.org>
>
> 	[BZ #5012]
> 	* FAQ.in: Describe why glibc needs to be compiled with
> 	optimization.
>
> ============================================================
> Index: FAQ.in
> --- FAQ.in	8 Apr 2005 20:59:50 -0000	1.137
> +++ FAQ.in	27 Oct 2007 15:14:50 -0000
> @@ -387,6 +387,32 @@ yourself. Please remember that for each 
>  patches required to get glibc HEAD into a runnable state. The best course
>  of action is to determine if you have all the required patches.
>  
> +??	Why do I get:
> +	`#error "glibc cannot be compiled without optimization"',
> +	when trying to compile GNU libc with GNU CC?
> +
> +{AJ,CO} There are a couple of reasons why the GNU C library will not work
> +correctly if it is not complied with optimzation.
> +
> +In the early startup of the dynamic loader (_dl_start), before
> +relocation of the PLT, you cannot make function calls. You must inline
> +the functions you will use during early startup, or call compiler
> +builtins (__builtin_*).
> +
> +Without optimizations enabled GNU CC will not inline functions. The
> +early startup of the dynamic loader will make function calls via an
> +unrelocated PLT and crash. 
> +
> +Without auditing the dynamic linker code it would be difficult to remove
> +this requirement. 
> +
> +Another reason is that nested functions must be inlined in many cases to
> +avoid executable stacks.
> +
> +In practice there is no reason to compile without optimizations, therefore
> +we require that GNU libc be compiled with optimizations enabled.
> +
> +
>  ? Installation and configuration issues
>  
>  ??	Can I replace the libc on my Linux system with GNU libc?
> @@ -1685,6 +1711,7 @@ Answers were given by:
>  {AO} Alexandre Oliva, <aoliva@redhat.com>
>  {BH} Bruno Haible, <haible@clisp.cons.org>
>  {SM} Steven Munroe, <sjmunroe@us.ibm.com>
> +{CO} Carlos O'Donell, <carlos@systemhalted.org>
>  \f
>  Local Variables:
>   mode:outline

Andreas
-- 
 Andreas Jaeger, Director Platform/openSUSE, aj@suse.de
  SUSE LINUX Products GmbH, GF: Markus Rex, HRB 16746 (AG Nürnberg)
   Maxfeldstr. 5, 90409 Nürnberg, Germany
    GPG fingerprint = 93A3 365E CE47 B889 DF7F  FED1 389A 563C C272 A126

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Re: BZ 5012: Add FAQ entry for compiling with optimization

[Roland McGrath]

I think you can always commit FAQ changes without getting approval.
If there are inaccuracies introduced, we can correct them.


Thanks,
Roland

Re: BZ 5012: Add FAQ entry for compiling with optimization

[Ulrich Drepper]

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Andreas Jaeger wrote:
> Is it ok to commit this one?

I commented on this when it was submitted.  This is not enough and the
proposed text is somewhat misleading.

- --
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖
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Re: BZ 5012: Add FAQ entry for compiling with optimization

[Andreas Jaeger]

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Ulrich Drepper <drepper@redhat.com> writes:

> Andreas Jaeger wrote:
>> Is it ok to commit this one?
>
> I commented on this when it was submitted.  This is not enough and the
> proposed text is somewhat misleading.

AFAIR your comments where on the first draft of this by Carlos and I
made changes to the text for this final form.

I cannot find any comment from you on the version that is referenced
here,

Andreas
-- 
 Andreas Jaeger, Director Platform/openSUSE, aj@suse.de
  SUSE LINUX Products GmbH, GF: Markus Rex, HRB 16746 (AG Nürnberg)
   Maxfeldstr. 5, 90409 Nürnberg, Germany
    GPG fingerprint = 93A3 365E CE47 B889 DF7F  FED1 389A 563C C272 A126

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