shttpd/sHT/compiler.h

/* s2 - architecture-independent compiler magic
   Copyright (C) 2018 Ariadne Devos

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>. */

#ifndef _sHT_COMPILER_H
#define _sHT_COMPILER_H

/* Clang or GCC is assumed, see Bug #3 */

#define sHT_likely(b) (__builtin_expect(!!(b), 1))
#define sHT_unlikely(b) (__builtin_expect(!!(b), 0))

/* Let the compiler forget the value of the variable var.
   This does not avoid speculation by the processor.
   For Spectre mitigations, consider sHT_despeculate. */
#define sHT_hide_var(var) \
	do { \
		__asm__("" : "=r" (var) : "0" (var)); \
	} while (0)

/** Let the compiler believe that value of the variable @var{var} depends upon
  @var{dependency}.

  Afterwards, the value of @var{var} will be supposedly-dependant upon the
  value of @var{dependency}. */
#define sHT_depend(var, dependency) \
	do { \
		__asm__("" : "=X" (var) : "X" (var), "X" (dependency)); \
	} while (0)

/** Return @var{val} unchanged, supposedly-dependant upon the value of
  @var{dependency}.

  Afterwards, the value of @var{var} will be supposedly-dependant upon the
  value of @var{dependency}. */
#define sHT_depending(val, dependency) \
	({ __typeof__(val) _sHT_dep_val = (val); \
	sHT_depend(_sHT_dep_val, dependency); \
	_sHT_dep_val; })

/** Evaluate the boolean expression @var{cond}.

  @var{cond}: a boolean expression variable in @var{var}
  @var{var}: a valid value to test

  In contrast to a plain C @code{if}, the compiler will not
  assume the correct branch is taken on a speculative execution.

  Its speculative value will be returned. */
#define sHT_test_hidden(var, cond) \
	((cond) ? ({ sHT_hide_var(var); (_Bool) 1; }) \
	: ({ sHT_hide_var(var); (_Bool) 0; }))

/** Reverse taking a pointer to a member.

  @var{ptr}: a valid pointer, pointing to an instance of the member
    denoted by @var{member}.
  @var{type}: a C union or struct type, possibly as a typedef
  @var{member}: the name of a member of @var{type}

  This concept is taken from Linux.
  TODO: compatibility with Intel MMX and other pointer bounds checkers.

  Return a pointer to the @var{type} structure, of which a pointer to its
  field @var{member} is equal to @var{ptr}. */
#define sHT_container_of(ptr, type, member) \
  ((type *) (((char *) ptr) - offsetof(type, member)))

/** Something has gone wrong, stop the current process.

  This is only for catching bugs. In particular, it may speculatively
  be ignored. (Replacing a possibly repeating bug with a crashing
  speculation bug is, from a security perspective, probably fine.)

 @var{msg}: a literal string, describing what should have been the case. */
#define sHT_halt(msg) __builtin_trap();

#endif