Portable C arithmetic right shift

Right shifting a signed quantity will fill with bit signs ("arithmetic shift") on some machines and with 0-bits ("logical shift") on others.

— The C Programming Language, Kernighan and Ritchie

Signed right shifts have implementation-defined behaviour¹.

Common implementations choose to fill with sign bits (gcc¹, clang², msvc³).

That is a signed arithmetic right shift (sar).

Functions in sar.c twiddle bits to provide portable arithmetic right shifts.

sari(-5, 1) == -3; /* true */

They compile to small, branchless instructions. See for yourself on the Compiler Explorer.

Standard integer types are supported for pedantic C99 and C++11.

Use

#include "stdint.h" /* optional */
#include "sar.c"

sari(-63, 3); /* -8 */
sarl(-0x200000000L, 1); /* -0x100000000L */
sarfast32(-0xDECAF, 8); /* -0xDED */
sar8(15, 2); /* 3 */

Details

Arithmetic right shift functions defined by "sar.c" are of the form

static inline type sar##(type m, char n);

where type is any of:

signed char   /* sarc */
short         /* sars */
int           /* sari */
long          /* sarl */
long long     /* sarll */

/* if stdint.h is included */
int_fast8_t   /* sarfast8 */
int_fast16_t  /* sarfast16 */
int_fast32_t  /* sarfast32 */
int_fast64_t  /* sarfast64 */
int_least8_t  /* sarleast8 */
int_least16_t /* sarleast16 */
int_least32_t /* sarleast32 */
int_least64_t /* sarleast64 */
intmax_t      /* sarmax */

/* optionally included in stdint.h */
int8_t        /* sar8 */
int16_t       /* sar16 */
int32_t       /* sar32 */
int64_t       /* sar64 */
intptr_t      /* sarptr */

The following macros are also defined.

/* For the include guard. */
#define SAR_C

/* Macro to define the sar## function bodies. Not standalone. */
#define SARBODY(type, utype)

Alternatives which force logical shifts, then correct them back, are defined in "saru.c" with the prefixes saru## and SARU.

Test

make

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¹ "The result of E1 >> E2 is E1 right-shifted E2 bit positions. ... If E1 has a signed type and a negative value, the resulting value is implementation-defined.", ISO C N2176 6.5.7, p84-85

² "Signed ‘>>’ acts on negative numbers by sign extension.", GCC C Implementation-Defined Behavior 4.5 Integers

³ "GCC is currently the defacto-standard open source compiler today", Clang - Features and Goals

⁴ "the Microsoft C++ compiler uses the sign bit to fill vacated bit positions" Left Shift and Right Shift Operators (>> and <<)