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[gcc/doc] Improve nonnull attribute documentation

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Improve nonnull attribute documentation in a number of ways:

Reorganize discussion of effects into:
- effects for calls to functions with nonnull-marked parameters, and
- effects for function definitions with nonnull-marked parameters.
This makes it clear that -fno-delete-null-pointer-checks has no effect for
optimizations based on nonnull-marked parameters in function definitions
(see PR100404).

Mention -Wnonnull-compare.

gcc/ChangeLog:

2021-07-28  Tom de Vries  <tdevries@suse.de>

	PR middle-end/101665
	* doc/extend.texi (nonnull attribute): Improve documentation.
This commit is contained in:
Tom de Vries 2021-07-28 15:44:54 +02:00
parent 99b520f031
commit 7d8577dd46
1 changed files with 31 additions and 11 deletions
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gcc/doc/extend.texi
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@ -3488,17 +3488,37 @@ my_memcpy (void *dest, const void *src, size_t len)
@end smallexample
@noindent
causes the compiler to check that, in calls to @code{my_memcpy},
arguments @var{dest} and @var{src} are non-null. If the compiler
determines that a null pointer is passed in an argument slot marked
as non-null, and the @option{-Wnonnull} option is enabled, a warning
is issued. @xref{Warning Options}. Unless disabled by
the @option{-fno-delete-null-pointer-checks} option the compiler may
also perform optimizations based on the knowledge that certain function
arguments cannot be null. In addition,
the @option{-fisolate-erroneous-paths-attribute} option can be specified
to have GCC transform calls with null arguments to non-null functions
into traps. @xref{Optimize Options}.
informs the compiler that, in calls to @code{my_memcpy}, arguments
@var{dest} and @var{src} must be non-null.
The attribute has an effect both on functions calls and function definitions.
For function calls:
@itemize @bullet
@item If the compiler determines that a null pointer is
passed in an argument slot marked as non-null, and the
@option{-Wnonnull} option is enabled, a warning is issued.
@xref{Warning Options}.
@item The @option{-fisolate-erroneous-paths-attribute} option can be
specified to have GCC transform calls with null arguments to non-null
functions into traps. @xref{Optimize Options}.
@item The compiler may also perform optimizations based on the
knowledge that certain function arguments cannot be null. These
optimizations can be disabled by the
@option{-fno-delete-null-pointer-checks} option. @xref{Optimize Options}.
@end itemize
For function definitions:
@itemize @bullet
@item If the compiler determines that a function parameter that is
marked with nonnull is compared with null, and
@option{-Wnonnull-compare} option is enabled, a warning is issued.
@xref{Warning Options}.
@item The compiler may also perform optimizations based on the
knowledge that @code{nonnul} parameters cannot be null. This can
currently not be disabled other than by removing the nonnull
attribute.
@end itemize
If no @var{arg-index} is given to the @code{nonnull} attribute,
all pointer arguments are marked as non-null. To illustrate, the