include/rseq/compiler.h

   1 /* SPDX-License-Identifier: MIT */
   2 /* SPDX-FileCopyrightText: 2021 Mathieu Desnoyers <mathieu.desnoyers@efficios.com> */
   3
   4 /*
   5  * rseq/compiler.h
   6  *
   7  * Work-around asm goto compiler bugs.
   8  */
   9
  10 #ifndef _RSEQ_COMPILER_H
  11 #define _RSEQ_COMPILER_H
  12
  13 #if defined __cplusplus
  14 # include <type_traits> /* for std::remove_cv */
  15 #endif
  16
  17 #define rseq_likely(x)          __builtin_expect(!!(x), 1)
  18 #define rseq_unlikely(x)        __builtin_expect(!!(x), 0)
  19 #define rseq_barrier()          __asm__ __volatile__("" : : : "memory")
  20
  21 /*
  22  * Instruct the compiler to perform only a single access to a variable
  23  * (prohibits merging and refetching). The compiler is also forbidden to reorder
  24  * successive instances of RSEQ_ACCESS_ONCE(), but only when the compiler is aware of
  25  * particular ordering. Compiler ordering can be ensured, for example, by
  26  * putting two RSEQ_ACCESS_ONCE() in separate C statements.
  27  *
  28  * This macro does absolutely -nothing- to prevent the CPU from reordering,
  29  * merging, or refetching absolutely anything at any time.  Its main intended
  30  * use is to mediate communication between process-level code and irq/NMI
  31  * handlers, all running on the same CPU.
  32  */
  33 #define RSEQ_ACCESS_ONCE(x)     (*(__volatile__  __typeof__(x) *)&(x))
  34
  35 #define RSEQ_WRITE_ONCE(x, v)   __extension__ ({ RSEQ_ACCESS_ONCE(x) = (v); })
  36 #define RSEQ_READ_ONCE(x)       RSEQ_ACCESS_ONCE(x)
  37
  38 /*
  39  * gcc prior to 4.8.2 miscompiles asm goto.
  40  * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58670
  41  *
  42  * gcc prior to 8.1.0 miscompiles asm goto at O1.
  43  * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103908
  44  *
  45  * clang prior to version 13.0.1 miscompiles asm goto at O2.
  46  * https://github.com/llvm/llvm-project/issues/52735
  47  *
  48  * Work around these issues by adding a volatile inline asm with
  49  * memory clobber in the fallthrough after the asm goto and at each
  50  * label target.  Emit this for all compilers in case other similar
  51  * issues are found in the future.
  52  */
  53 #define rseq_after_asm_goto()   __asm__ __volatile__ ("" : : : "memory")
  54
  55 /* Combine two tokens. */
  56 #define RSEQ__COMBINE_TOKENS(_tokena, _tokenb)  \
  57         _tokena##_tokenb
  58 #define RSEQ_COMBINE_TOKENS(_tokena, _tokenb)   \
  59         RSEQ__COMBINE_TOKENS(_tokena, _tokenb)
  60
  61 #if defined(__SIZEOF_LONG__)
  62 #define RSEQ_BITS_PER_LONG      (__SIZEOF_LONG__ * 8)
  63 #elif defined(_LP64)
  64 #define RSEQ_BITS_PER_LONG      64
  65 #else
  66 #define RSEQ_BITS_PER_LONG      32
  67 #endif
  68
  69 #ifdef __cplusplus
  70 #define rseq_unqual_scalar_typeof(x)                                    \
  71         std::remove_cv<std::remove_reference<decltype(x)>::type>::type
  72 #else
  73 #define rseq_scalar_type_to_expr(type)                                  \
  74         unsigned type: (unsigned type)0,                                \
  75         signed type: (signed type)0
  76
  77 /*
  78  * Use C11 _Generic to express unqualified type from expression. This removes
  79  * volatile qualifier from expression type.
  80  */
  81 #define rseq_unqual_scalar_typeof(x)                                    \
  82         __typeof__(                                                     \
  83                 _Generic((x),                                           \
  84                         char: (char)0,                                  \
  85                         rseq_scalar_type_to_expr(char),                 \
  86                         rseq_scalar_type_to_expr(short),                \
  87                         rseq_scalar_type_to_expr(int),                  \
  88                         rseq_scalar_type_to_expr(long),                 \
  89                         rseq_scalar_type_to_expr(long long),            \
  90                         default: (x)                                    \
  91                 )                                                       \
  92         )
  93 #endif
  94
  95 #endif  /* _RSEQ_COMPILER_H */