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Clarify the <sys/ifunc.h> docs.

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This confused the Arm folks implementing function multi-versioning.

Test: treehugger
Change-Id: Ib56fc142bed4a2f2a394d60a07f322add40702e5
This commit is contained in:
Elliott Hughes 2023-07-10 22:32:09 +00:00
parent 143f3cea32
commit 0c50ed4c7c
1 changed files with 16 additions and 9 deletions
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25
libc/include/sys/ifunc.h
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@ -40,13 +40,15 @@ __BEGIN_DECLS
#if defined(__aarch64__)
/**
* Provides information about hardware capabilities to ifunc resolvers.
* Provides information about hardware capabilities to arm64 ifunc resolvers.
*
* Starting with API level 30, ifunc resolvers on arm64 are passed two arguments. The first is a
* uint64_t whose value is equal to getauxval(AT_HWCAP) | _IFUNC_ARG_HWCAP. The second is a pointer
* to a data structure of this type. Prior to API level 30, no arguments are passed to ifunc
* resolvers. Code that wishes to be compatible with prior API levels should not accept any
* arguments in the resolver.
* Prior to API level 30, arm64 ifunc resolvers are passed no arguments.
*
* Starting with API level 30, arm64 ifunc resolvers are passed two arguments.
* The first is a uint64_t whose value is equal to getauxval(AT_HWCAP) | _IFUNC_ARG_HWCAP.
* The second is a pointer to a data structure of this type.
*
* Code that wishes to be compatible with API levels before 30 must call getauxval() itself.
*/
typedef struct __ifunc_arg_t {
/** Set to sizeof(__ifunc_arg_t). */
@ -60,9 +62,14 @@ typedef struct __ifunc_arg_t {
} __ifunc_arg_t;
/**
* If this bit is set in the first argument to an ifunc resolver, indicates that the second argument
* is a pointer to a data structure of type __ifunc_arg_t. This bit is always set on Android
* starting with API level 30.
* If this bit is set in the first argument to an ifunc resolver, the second argument
* is a pointer to a data structure of type __ifunc_arg_t.
*
* This bit is always set on Android starting with API level 30.
* This bit is meaningless before API level 30 because ifunc resolvers are not passed any arguments.
* This bit has no real use on Android, but is included for glibc source compatibility;
* glibc used this bit to distinguish the case where the ifunc resolver received a single argument,
* which was an evolutionary stage Android never went through.
*/
#define _IFUNC_ARG_HWCAP (1ULL << 62)