Changes since v4: hopefully addressed all of Sandra's requests and suggestions concerning the documentation snippets, thanks for the feedback. If it still isn't clear, feel free to rephrase -- I'm a programmer, not a technical writer.
2017-01-13 Torsten Duwe <d...@suse.de> : * c-family/c-attribs.c : introduce prolog_pad attribute and create a handler for it. * lto/lto-lang.c : Likewise. * common.opt : introduce -fprolog_pad command line option and its variables prolog_nop_pad_size and prolog_nop_pad_entry. * doc/extend.texi : document prolog_pad attribute. * doc/invoke.texi : document -fprolog_pad command line option. * opts.c (OPT_fprolog_pad_): add parser. * doc/tm.texi.in (TARGET_ASM_PRINT_PROLOG_PAD): new target hook * doc/tm.texi : Likewise. * target.def (print_prolog_pad): Likewise. * targhooks.h (default_print_prolog_pad): new function. * targhooks.c (default_print_prolog_pad): Likewise. * testsuite/c-c++-common/attribute-prolog_pad-1.c : New test. * toplev.c (process_options): Switch off IPA-RA if prolog pads are being generated. * varasm.c (assemble_start_function): look at prolog-pad command line switch and function attributes and maybe generate NOP pads by calling print_prolog_pad. diff --git a/gcc/c-family/c-attribs.c b/gcc/c-family/c-attribs.c index ce7fcaa..5c6cf1c 100644 --- a/gcc/c-family/c-attribs.c +++ b/gcc/c-family/c-attribs.c @@ -139,6 +139,7 @@ static tree handle_bnd_variable_size_attribute (tree *, tree, tree, int, bool *) static tree handle_bnd_legacy (tree *, tree, tree, int, bool *); static tree handle_bnd_instrument (tree *, tree, tree, int, bool *); static tree handle_fallthrough_attribute (tree *, tree, tree, int, bool *); +static tree handle_prolog_pad_attribute (tree *, tree, tree, int, bool *); /* Table of machine-independent attributes common to all C-like languages. @@ -345,6 +346,8 @@ const struct attribute_spec c_common_attribute_table[] = handle_bnd_instrument, false }, { "fallthrough", 0, 0, false, false, false, handle_fallthrough_attribute, false }, + { "prolog_pad", 1, 2, true, false, false, + handle_prolog_pad_attribute, false }, { NULL, 0, 0, false, false, false, NULL, false } }; @@ -3173,3 +3176,10 @@ handle_fallthrough_attribute (tree *, tree name, tree, int, *no_add_attrs = true; return NULL_TREE; } + +static tree +handle_prolog_pad_attribute (tree *, tree, tree, int, + bool *) +{ + return NULL_TREE; +} diff --git a/gcc/common.opt b/gcc/common.opt index 8ad5b77..37d4009 100644 --- a/gcc/common.opt +++ b/gcc/common.opt @@ -163,6 +163,13 @@ bool flag_stack_usage_info = false Variable int flag_debug_asm +; If we should generate NOP pads before each function prologue +Variable +HOST_WIDE_INT prolog_nop_pad_size + +; And how far the asm entry point is into this pad +Variable +HOST_WIDE_INT prolog_nop_pad_entry ; Balance between GNAT encodings and standard DWARF to emit. Variable @@ -2019,6 +2026,10 @@ fprofile-reorder-functions Common Report Var(flag_profile_reorder_functions) Enable function reordering that improves code placement. +fprolog-pad= +Common Joined Optimization +Pad NOPs before each function prologue. + frandom-seed Common Var(common_deferred_options) Defer diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi index 6be113c..fb7d62d 100644 --- a/gcc/doc/extend.texi +++ b/gcc/doc/extend.texi @@ -3076,6 +3076,23 @@ that affect more than one function. This attribute should be used for debugging purposes only. It is not suitable in production code. +@item prolog_pad +@cindex @code{prolog_pad} function attribute +@cindex function entry padded with NOPs +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. Usually, +prolog padding is enabled globally using the @option{-fprolog-pad=N,M} +command-line switch, and disabled with attribute @code{prolog_pad (0)} +for functions that are part of the actual instrumentation framework. +This conveniently avoids an endless recursion. +Of course the @code{prolog_pad} function attribute can be used to +change the pad size to any desired value. The two-value syntax is +the same as for the command-line switch @option{-fprolog-pad=N,M}, +generating a NOP pad of size @var{N}, with the function entry point +@var{M} NOP instructions into the pad. @var{M} defaults to 0 +if omitted e.g. function entry point is the beginning of the pad. + @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 69f0adb..d2e11bc 100644 --- a/gcc/doc/invoke.texi +++ b/gcc/doc/invoke.texi @@ -11341,6 +11341,27 @@ 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 -fprolog-pad=@var{N},@var{M} +@opindex fprolog-pad +Generate a pad of @var{N} NOPs right at the beginning +of each function, with the function entry point @var{M} NOPs into +the pad, which can be used to patch in any desired +instrumentation at run time, provided that the code segment +is writable. +For run-time identification, the starting addresses +of these pads, which correspond to their respective function entries +minus @var{M}, are additionally collected in the @code{__prolog_pads_loc} +section of the resulting binary. + +Note that value of @code{__attribute__ ((prolog_pad (N,M)))} takes +precedence over command-line option @option{-fprolog-pad=N,M}. +This can be used to increase the pad size or to remove the pad completely +on a single function. If @code{N=0}, no pad location is recorded. + +The NOP pad is inserted at (and maybe before) the function entry address, +even before the prologue. If @var{M} is omitted, it defaults to 0 e.g. the +pad starts exactly at the function entry point and not before. + @end table diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi index 4b62e05..eb29f71 100644 --- a/gcc/doc/tm.texi +++ b/gcc/doc/tm.texi @@ -4564,6 +4564,16 @@ 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_PROLOG_PAD (FILE *@var{file}, unsigned HOST_WIDE_INT @var{pad_size}, bool @var{record_p}) +Generate an array of @var{pad_size} NOP instructions in the +output @var{file}. If the target supports NOPs of different sizes, +the instruction with the smallest code size should be used, in order +to achieve the finest granularity for @var{pad_size}. If @var{record_p} +is true, the current location should be recorded in the +@code{__prolog_pads_loc} section or a target-specific equivalent. + +@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 ea74d37..7b01026 100644 --- a/gcc/doc/tm.texi.in +++ b/gcc/doc/tm.texi.in @@ -3648,6 +3648,8 @@ 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_PROLOG_PAD + @hook TARGET_ASM_FUNCTION_PROLOGUE @hook TARGET_ASM_FUNCTION_END_PROLOGUE diff --git a/gcc/lto/lto-lang.c b/gcc/lto/lto-lang.c index fccb8c6..d4955c0 100644 --- a/gcc/lto/lto-lang.c +++ b/gcc/lto/lto-lang.c @@ -50,6 +50,7 @@ static tree handle_sentinel_attribute (tree *, tree, tree, int, bool *); static tree handle_type_generic_attribute (tree *, tree, tree, int, bool *); static tree handle_transaction_pure_attribute (tree *, tree, tree, int, bool *); static tree handle_returns_twice_attribute (tree *, tree, tree, int, bool *); +static tree handle_prolog_pad_attribute (tree *, tree, tree, int, bool *); static tree ignore_attribute (tree *, tree, tree, int, bool *); static tree handle_format_attribute (tree *, tree, tree, int, bool *); @@ -78,6 +79,8 @@ const struct attribute_spec lto_attribute_table[] = handle_nonnull_attribute, false }, { "nothrow", 0, 0, true, false, false, handle_nothrow_attribute, false }, + { "prolog_pad", 1, 2, true, false, false, + handle_prolog_pad_attribute, false }, { "returns_twice", 0, 0, true, false, false, handle_returns_twice_attribute, false }, { "sentinel", 0, 1, false, true, true, @@ -475,6 +478,14 @@ handle_returns_twice_attribute (tree *node, tree ARG_UNUSED (name), return NULL_TREE; } +static tree +handle_prolog_pad_attribute (tree *, tree, tree, int, + bool *) +{ + /* Nothing to be done here. */ + return NULL_TREE; +} + /* Ignore the given attribute. Used when this attribute may be usefully overridden by the target, but is not used generically. */ diff --git a/gcc/opts.c b/gcc/opts.c index 5f573a1..60d80aa 100644 --- a/gcc/opts.c +++ b/gcc/opts.c @@ -2157,6 +2157,26 @@ common_handle_option (struct gcc_options *opts, opts->x_flag_ipa_reference = false; break; + case OPT_fprolog_pad_: + { + const char *comma = strchr (arg, ','); + if (comma) + { + prolog_nop_pad_size = atoi (arg); + prolog_nop_pad_entry = atoi (comma + 1); + } + else + { + prolog_nop_pad_size = atoi (arg); + prolog_nop_pad_entry = 0; + } + if (prolog_nop_pad_size < 0 + || prolog_nop_pad_entry < 0 + || prolog_nop_pad_size < prolog_nop_pad_entry) + error ("invalid arguments for %<-fprolog_pad%>"); + } + break; + case OPT_ftree_vectorize: if (!opts_set->x_flag_tree_loop_vectorize) opts->x_flag_tree_loop_vectorize = value; diff --git a/gcc/target.def b/gcc/target.def index 0443390..60e56cd 100644 --- a/gcc/target.def +++ b/gcc/target.def @@ -288,6 +288,12 @@ hidden, protected or internal visibility as specified by @var{visibility}.", void, (tree decl, int visibility), default_assemble_visibility) +DEFHOOK +(print_prolog_pad, + "Generate prologue pad", + void, (FILE *file, unsigned HOST_WIDE_INT pad_size, bool record_p), + default_print_prolog_pad) + /* Output the assembler code for entry to a function. */ DEFHOOK (function_prologue, diff --git a/gcc/targhooks.c b/gcc/targhooks.c index 2f2abd3..2ee5e38 100644 --- a/gcc/targhooks.c +++ b/gcc/targhooks.c @@ -1617,6 +1617,31 @@ default_compare_by_pieces_branch_ratio (machine_mode) return 1; } +/* Write pad_size NOPs into the asm outfile before a function + prologue. If record_p is true, the location of the pad will be + recorded in a special object section called "__prolog_pads_loc". + This routine may be called twice per function to put NOPs before + and after the function entry. */ + +void +default_print_prolog_pad (FILE *file, unsigned HOST_WIDE_INT pad_size, + bool record_p) +{ + if (record_p) + fprintf (file, "1:"); + + unsigned i; + for (i = 0; i < pad_size; ++i) + fprintf (file, "\tnop\n"); + + if (record_p) + { + fprintf (file, "\t.section __prolog_pads_loc, \"a\",@progbits\n"); + fprintf (file, "\t.quad 1b\n"); + fprintf (file, "\t.previous\n"); + } +} + bool default_profile_before_prologue (void) { diff --git a/gcc/targhooks.h b/gcc/targhooks.h index a5565f5..e302e8d 100644 --- a/gcc/targhooks.h +++ b/gcc/targhooks.h @@ -203,6 +203,7 @@ extern bool default_use_by_pieces_infrastructure_p (unsigned HOST_WIDE_INT, bool); extern int default_compare_by_pieces_branch_ratio (machine_mode); +extern void default_print_prolog_pad (FILE *, unsigned HOST_WIDE_INT , bool); extern bool default_profile_before_prologue (void); extern reg_class_t default_preferred_reload_class (rtx, reg_class_t); extern reg_class_t default_preferred_output_reload_class (rtx, reg_class_t); diff --git a/gcc/testsuite/c-c++-common/attribute-prolog_pad-1.c b/gcc/testsuite/c-c++-common/attribute-prolog_pad-1.c new file mode 100644 index 0000000..2236aa8 --- /dev/null +++ b/gcc/testsuite/c-c++-common/attribute-prolog_pad-1.c @@ -0,0 +1,34 @@ +/* { dg-do compile } */ +/* { dg-options "-fprolog-pad=3,1" } */ + +void f1 (void) __attribute__((prolog_pad(2,1))); +void f2 (void) __attribute__((prolog_pad(3))); +int f3 (void); + +void +f1 (void) +{ + f2 (); +} + +void +f2 (void) +{ + f1 (); +} + +/* F3 should never have a NOP pad. */ +int +__attribute__((prolog_pad(0))) +__attribute__((noinline)) +f3 (void) +{ + return 5; +} + +/* F4 should receive the command line default setting. */ +int +f4 (void) +{ + return 3*f3 ()+1; +} diff --git a/gcc/toplev.c b/gcc/toplev.c index c0f1a2d..649c4bd 100644 --- a/gcc/toplev.c +++ b/gcc/toplev.c @@ -1595,8 +1595,10 @@ process_options (void) } /* Do not use IPA optimizations for register allocation if profiler is active + or prolog pads are inserted for run-time instrumentation or port does not emit prologue and epilogue as RTL. */ - if (profile_flag || !targetm.have_prologue () || !targetm.have_epilogue ()) + if (profile_flag || prolog_nop_pad_size + || !targetm.have_prologue () || !targetm.have_epilogue ()) flag_ipa_ra = 0; /* Enable -Werror=coverage-mismatch when -Werror and -Wno-error diff --git a/gcc/varasm.c b/gcc/varasm.c index 11a8ac4..6c40bce 100644 --- a/gcc/varasm.c +++ b/gcc/varasm.c @@ -1830,6 +1830,44 @@ assemble_start_function (tree decl, const char *fnname) if (DECL_PRESERVE_P (decl)) targetm.asm_out.mark_decl_preserved (fnname); + unsigned HOST_WIDE_INT pad_size = prolog_nop_pad_size; + unsigned HOST_WIDE_INT pad_entry = prolog_nop_pad_entry; + + tree prolog_pad_attr + = lookup_attribute ("prolog_pad", DECL_ATTRIBUTES (decl)); + if (prolog_pad_attr) + { + tree pp_val = TREE_VALUE (prolog_pad_attr); + tree prolog_pad_value1 = TREE_VALUE (pp_val); + + if (tree_fits_uhwi_p (prolog_pad_value1)) + pad_size = tree_to_uhwi (prolog_pad_value1); + else + gcc_unreachable (); + + pad_entry = 0; + if (list_length (pp_val) > 1) + { + tree prolog_pad_value2 = TREE_VALUE (TREE_CHAIN (pp_val)); + + if (tree_fits_uhwi_p (prolog_pad_value2)) + pad_entry = tree_to_uhwi (prolog_pad_value2); + else + gcc_unreachable (); + } + } + + if (pad_entry > pad_size) + { + if (pad_size > 0) + warning (OPT_Wattributes, "Prolog nop pad entry > size"); + pad_entry = 0; + } + + /* Emit the prolog padding before the entry label, if any. */ + if (pad_entry > 0) + targetm.asm_out.print_prolog_pad (asm_out_file, pad_entry, true); + /* Do any machine/system dependent processing of the function name. */ #ifdef ASM_DECLARE_FUNCTION_NAME ASM_DECLARE_FUNCTION_NAME (asm_out_file, fnname, current_function_decl); @@ -1838,6 +1876,11 @@ assemble_start_function (tree decl, const char *fnname) ASM_OUTPUT_FUNCTION_LABEL (asm_out_file, fnname, current_function_decl); #endif /* ASM_DECLARE_FUNCTION_NAME */ + /* And the padding after the label. Record it if we haven't done so yet. */ + if (pad_size > pad_entry) + targetm.asm_out.print_prolog_pad (asm_out_file, pad_size-pad_entry, + (pad_entry == 0)); + if (lookup_attribute ("no_split_stack", DECL_ATTRIBUTES (decl))) saw_no_split_stack = true; }