On 03/14/2016 06:15 PM, Sandra Loosemore wrote:
> On 03/14/2016 12:40 PM, Carlos O'Donell wrote:
>> Using the 'leaf' attribute is difficult in certain use cases, and
>> the documentation rightly points out that signals is one such
>> problem.
>>
>> We should additionally document the following caveats:
>>
>> * Indirect function resolvers (thanks to Florian Weimer for
>> catching this). * Indirect function implementations * ELF symbol
>> interposition.
>>
>> [snip]
>>
>> gcc/ 2016-03-14 Carlos O'Donell <car...@redhat.com>
>>
>> * doc/extend.texi (Common Function Attributes): Describe ifunc
>> impact on leaf attribute.
>>
>
> Hmmmm. Both your patch and the original text really need some
> copy-editing to fix noun/verb agreement, punctuation, etc. How about
> something like the attached patch? I just threw this together and
> haven't tested this in any way, but you confirm that it builds and it
> looks OK to you, feel free to check it in.
PDF looks good.
Committed as r234247.
2016-03-16 Carlos O'Donell <car...@redhat.com>
Sandra Loosemore <san...@codesourcery.com>
* doc/extend.texi (Common Function Attributes): Describe ifunc impact
on leaf attribute. Mention ELF interposition problems.
Index: gcc/doc/extend.texi
===================================================================
--- gcc/doc/extend.texi (revision 234236)
+++ gcc/doc/extend.texi (revision 234247)
@@ -2772,30 +2772,41 @@
@item leaf
@cindex @code{leaf} function attribute
-Calls to external functions with this attribute must return to the current
-compilation unit only by return or by exception handling. In particular, leaf
-functions are not allowed to call callback function passed to it from the
current
-compilation unit or directly call functions exported by the unit or longjmp
-into the unit. Leaf function might still call functions from other compilation
-units and thus they are not necessarily leaf in the sense that they contain no
-function calls at all.
+Calls to external functions with this attribute must return to the
+current compilation unit only by return or by exception handling. In
+particular, a leaf function is not allowed to invoke callback functions
+passed to it from the current compilation unit, directly call functions
+exported by the unit, or @code{longjmp} into the unit. Leaf functions
+might still call functions from other compilation units and thus they
+are not necessarily leaf in the sense that they contain no function
+calls at all.
-The attribute is intended for library functions to improve dataflow analysis.
-The compiler takes the hint that any data not escaping the current compilation
unit can
-not be used or modified by the leaf function. For example, the @code{sin}
function
-is a leaf function, but @code{qsort} is not.
+The attribute is intended for library functions to improve dataflow
+analysis. The compiler takes the hint that any data not escaping the
+current compilation unit cannot be used or modified by the leaf
+function. For example, the @code{sin} function is a leaf function, but
+@code{qsort} is not.
-Note that leaf functions might invoke signals and signal handlers might be
-defined in the current compilation unit and use static variables. The only
-compliant way to write such a signal handler is to declare such variables
-@code{volatile}.
+Note that leaf functions might indirectly run a signal handler defined
+in the current compilation unit that uses static variables. Similarly,
+when lazy symbol resolution is in effect, leaf functions might invoke
+indirect functions whose resolver function or implementation function is
+defined in the current compilation unit and uses static variables. There
+is no standard-compliant way to write such a signal handler, resolver
+function, or implementation function, and the best that you can do is to
+remove the @code{leaf} attribute or mark all such static variables
+@code{volatile}. Lastly, for ELF-based systems that support symbol
+interposition, care should be taken that functions defined in the
+current compilation unit do not unexpectedly interpose other symbols
+based on the defined standards mode and defined feature test macros;
+otherwise an inadvertent callback would be added.
-The attribute has no effect on functions defined within the current compilation
-unit. This is to allow easy merging of multiple compilation units into one,
-for example, by using the link-time optimization. For this reason the
-attribute is not allowed on types to annotate indirect calls.
+The attribute has no effect on functions defined within the current
+compilation unit. This is to allow easy merging of multiple compilation
+units into one, for example, by using the link-time optimization. For
+this reason the attribute is not allowed on types to annotate indirect
+calls.
-
@item malloc
@cindex @code{malloc} function attribute
@cindex functions that behave like malloc
--
Cheers,
Carlos.
