diff options
Diffstat (limited to 'gcc/doc/gty.texi')
| -rw-r--r-- | gcc/doc/gty.texi | 29 |
1 files changed, 18 insertions, 11 deletions
diff --git a/gcc/doc/gty.texi b/gcc/doc/gty.texi index 6d4f8394f42..56ce597f8dc 100644 --- a/gcc/doc/gty.texi +++ b/gcc/doc/gty.texi @@ -132,8 +132,19 @@ field really isn't ever used. @itemx default The type machinery needs to be told which field of a @code{union} is -currently active. This is done by giving each field a constant @code{tag} -value, and then specifying a discriminator using @code{desc}. For example, +currently active. This is done by giving each field a constant +@code{tag} value, and then specifying a discriminator using @code{desc}. +The value of the expression given by @code{desc} is compared against +each @code{tag} value, each of which should be different. If no +@code{tag} is matched, the field marked with @code{default} is used if +there is one, otherwise no field in the union will be marked. + +In the @code{desc} option, the ``current structure'' is the union that +it discriminates. Use @code{%1} to mean the structure containing it. +(There are no escapes available to the @code{tag} option, since it's +supposed to be a constant.) + +For example, @smallexample struct tree_binding GTY(()) @{ @@ -141,19 +152,15 @@ struct tree_binding GTY(()) union tree_binding_u @{ tree GTY ((tag ("0"))) scope; struct cp_binding_level * GTY ((tag ("1"))) level; - @} GTY ((desc ("BINDING_HAS_LEVEL_P ((tree)&%0)"))) scope; + @} GTY ((desc ("BINDING_HAS_LEVEL_P ((tree)&%0)"))) xscope; tree value; @}; @end smallexample -In the @code{desc} option, the ``current structure'' is the union that -it discriminates. Use @code{%1} to mean the structure containing it. -(There are no escapes available to the @code{tag} option, since it's -supposed to be a constant.) - -Each @code{tag} should be different. If no @code{tag} is matched, -the field marked with @code{default} is used if there is one, otherwise -no field in the union will be marked. +In this example, the value of BINDING_HAS_LEVEL_P when applied to a +@code{struct tree_binding *} is presumed to be 0 or 1. If 1, the type +mechanism will treat the field @code{level} as being present and if 0, +will treat the field @code{scope} as being present. @findex param_is @findex use_param |