Hi, 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. Mention workaround from PR100404 comment 7. The workaround can be used for this scenario. Say we have a test.c: ... #include <stdlib.h> extern int isnull (char *ptr) __attribute__ ((nonnull)); int isnull (char *ptr) { if (ptr == 0) return 1; return 0; } int main (void) { char *ptr = NULL; if (isnull (ptr)) __builtin_abort (); return 0; } ... The test-case contains a mistake: ptr == NULL, and we want to detect the mistake using an abort: ... $ gcc test.c $ ./a.out Aborted (core dumped) ... At -O2 however, the mistake is not detected: ... $ gcc test.c -O2 $ ./a.out ... which is what -Wnonnull-compare (not show here) warns about. The easiest way to fix this is by dropping the nonnull attribute. But that also disables -Wnonnull, which would detect something like: ... if (isnull (NULL)) __builtin_abort (); ... at compile time. Using this workaround: ... int isnull (char *ptr) { + asm ("" : "+r"(ptr)); if (ptr == 0) return 1; return 0; } ... we still manage to detect the problem at runtime with -O2: ... $ ~/gcc_versions/devel/install/bin/gcc test.c -O2 $ ./a.out Aborted (core dumped) ... while keeping the possibility to detect "isnull (NULL)" at compile time. OK for trunk? Thanks, - Tom [gcc/doc] Improve nonnull attribute documentation gcc/ChangeLog: 2021-07-28 Tom de Vries <tdevr...@suse.de> * doc/extend.texi (nonnull attribute): Improve documentation. --- gcc/doc/extend.texi | 51 ++++++++++++++++++++++++++++++++++++++++----------- 1 file changed, 40 insertions(+), 11 deletions(-) diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi index b83cd4919bb..3389effd70c 100644 --- a/gcc/doc/extend.texi +++ b/gcc/doc/extend.texi @@ -3488,17 +3488,46 @@ 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 effect both for 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 non-null 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 certain function parameters cannot be null. This can +be disabled by hiding the nonnullness using an inline assembly statement: + +@smallexample +extern int isnull (char *ptr) __attribute__((nonnull)); +int isnull (char *ptr) @{ + asm ("" : "+r"(ptr)); + if (ptr == 0) + return 1; + return 0; +@} +@end smallexample +@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