c-attribs.c (c_common_attribute_table): Add entry for "patchable_function_entry".

2017-07-07  Torsten Duwe  <duwe@suse.de>

        c-family/
	* c-attribs.c (c_common_attribute_table): Add entry for
	"patchable_function_entry".

        lto/
	* lto-lang.c (lto_attribute_table): Add entry for
	"patchable_function_entry".

	* common.opt: Introduce -fpatchable-function-entry
	command line option, and its variables function_entry_patch_area_size
	and function_entry_patch_area_start.
	* opts.c (common_handle_option): Add -fpatchable_function_entry_ case,
	including a two-value parser.
	* target.def (print_patchable_function_entry): New target hook.
	* targhooks.h (default_print_patchable_function_entry): New function.
	* targhooks.c (default_print_patchable_function_entry): Likewise.
	* toplev.c (process_options): Switch off IPA-RA if
	patchable function entries are being generated.
	* varasm.c (assemble_start_function): Look at the
	patchable-function-entry command line switch and current
	function attributes and maybe generate NOP instructions by
	calling the print_patchable_function_entry hook.
	* doc/extend.texi: Document patchable_function_entry attribute.
	* doc/invoke.texi: Document -fpatchable_function_entry
	command line option.
	* doc/tm.texi.in (TARGET_ASM_PRINT_PATCHABLE_FUNCTION_ENTRY):
	New target hook.
	* doc/tm.texi: Re-generate.

	* c-c++-common/patchable_function_entry-default.c: New test.
	* c-c++-common/patchable_function_entry-decl.c: Likewise.
	* c-c++-common/patchable_function_entry-definition.c: Likewise.

From-SVN: r250521

4 files changed, 60 insertions, 0 deletions

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 1dd5375..562666e 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 you must disable
+instrumentation on all functions that are part of the instrumentation
+framework with the attribute @code{patchable_function_entry (0)}
+to prevent recursion.
+
 @item pure
 @cindex @code{pure} function attribute
 @cindex functions that have no side effects
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 4926c90..a6da37c 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -11519,6 +11519,34 @@ 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.
+
+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.
+
 @end table
 
 
diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index 795e492..23e85c7 100644
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -4573,6 +4573,15 @@ will select the smallest suitable mode.
 This section describes the macros that output function entry
 (@dfn{prologue}) and exit (@dfn{epilogue}) code.
 
+@deftypefn {Target Hook} void TARGET_ASM_PRINT_PATCHABLE_FUNCTION_ENTRY (FILE *@var{file}, unsigned HOST_WIDE_INT @var{patch_area_size}, bool @var{record_p})
+Generate a patchable area at the function start, consisting of
+@var{patch_area_size} NOP instructions.  If the target supports named
+sections and if @var{record_p} is true, insert a pointer to the current
+location in the table of patchable functions.  The default implementation
+of the hook places the table of pointers in the special section named
+@code{__patchable_function_entries}.
+@end deftypefn
+
 @deftypefn {Target Hook} void TARGET_ASM_FUNCTION_PROLOGUE (FILE *@var{file}, HOST_WIDE_INT @var{size})
 If defined, a function that outputs the assembler code for entry to a
 function.  The prologue is responsible for setting up the stack frame,
diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in
index 98f2e6b..6df08a2 100644
--- a/gcc/doc/tm.texi.in
+++ b/gcc/doc/tm.texi.in
@@ -3650,6 +3650,8 @@ will select the smallest suitable mode.
 This section describes the macros that output function entry
 (@dfn{prologue}) and exit (@dfn{epilogue}) code.
 
+@hook TARGET_ASM_PRINT_PATCHABLE_FUNCTION_ENTRY
+
 @hook TARGET_ASM_FUNCTION_PROLOGUE
 
 @hook TARGET_ASM_FUNCTION_END_PROLOGUE