On 07/05/2017 01:36 AM, Torsten Duwe wrote:
Changes since v9:
* Do not store (declare static) the nop pattern template string.
In the future, it might depend on the particular function
being emitted. Fetch it freshly each time instead.
* On platforms without named sections, simply omit the recording
of the nop locations. Run-time instrumentation can still fiddle
it out, if desired. Document this behaviour in a half sentence.
* Move the hook documentation to where it belongs. Texi file (re-)
generation should work cleanly now.
* Documentation clarified as requested.
Torsten
[snip]
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 03ba8fc436c..a4c3c98b9f5 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -3105,6 +3105,27 @@ that affect more than one function.
This attribute should be used for debugging purposes only. It is not
suitable in production code.
+@item patchable_function_entry
+@cindex @code{patchable_function_entry} function attribute
+@cindex extra NOP instructions at the function entry point
+In case the target's text segment can be made writable at run time by
+any means, padding the function entry with a number of NOPs can be
+used to provide a universal tool for instrumentation.
+
+The @code{patchable_function_entry} function attribute can be used to
+change the number of NOPs to any desired value. The two-value syntax
+is the same as for the command-line switch
+@option{-fpatchable-function-entry=N,M}, generating @var{N} NOPs, with
+the function entry point before the @var{M}th NOP instruction.
+@var{M} defaults to 0 if omitted e.g. function entry point is before
+the first NOP.
+
+If patchable function entries are enabled globally using the command
+line option @option{-fpatchable-function-entry=N,M}, then all functions
s/command line option/command-line option
+that are part of the instrumentation framework must disable
+instrumentation with the attribute @code{patchable_function_entry (0)}
+to prevent recursion.
The functions don't disable instrumentation, programmers disable the
instrumentation on functions. Rewrite this clause as
...then you must disable instrumentation on all functions that are part
of the instrumentation framework with the attribute...
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 04cecf94405..1b8a4555b33 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -11520,6 +11520,34 @@ of the function name, it is considered to be a match.
For C99 and C++
extended identifiers, the function name must be given in UTF-8, not
using universal character names.
+@item -fpatchable-function-entry=@var{N}[,@var{M}]
+@opindex fpatchable-function-entry
+Generate @var{N} NOPs right at the beginning
+of each function, with the function entry point before the @var{M}th NOP.
+If @var{M} is omitted, it defaults to @code{0} so the
+function entry points to the address just at the first NOP.
+The NOP instructions reserve extra space which can be used to patch in
+any desired instrumentation at run time, provided that the code segment
+is writable. The amount of space is controllable indirectly via
+the number of NOPs; the NOP instruction used corresponds to the instruction
+emitted by the internal GCC back-end interface @code{gen_nop}. This behavior
+is target-specific and may also depend on the architecture variant and/or
+other compilation options.
+
+For run-time identification, the starting addresses of these areas,
+which correspond to their respective function entries minus @var{M},
+are additionally collected in the @code{__patchable_function_entries}
+section of the resulting binary, if the platform supports it.
+
+Note that the value of @code{__attribute__ ((patchable_function_entry
+(N,M)))} takes precedence over command-line option
+@option{-fpatchable-function-entry=N,M}. This can be used to increase
+the area size or to remove it completely on a single function.
+If @code{N=0}, no pad location is recorded.
+
+The NOP instructions are inserted at --- and maybe before, depending on
+@var{M} --- the function entry address, even before the prologue.
No spaces around the em-dashes '---'.
+DEFHOOK
+(print_patchable_function_entry,
+ "Generate a patchable area at the function start, consisting of\n\
+@var{patch_area_size} NOP instructions. If the target supports named\n\
+sections and if @var{record_p} is true, insert a pointer to the current\n\
+location in the table of patchable functions. This table will then be held\n\
+in a special section called @code{__patchable_function_entries}.",
I don't understand when "then" might be. Can you rewrite this in the
present tense? "This table is held..."
-Sandra
