linux/arch/mips/include/asm/unroll.h

/* SPDX-License-Identifier: GPL-2.0-only */
#ifndef __ASM_UNROLL_H__
#define __ASM_UNROLL_H__

/*
 * Explicitly unroll a loop, for use in cases where doing so is performance
 * critical.
 *
 * Ideally we'd rely upon the compiler to provide this but there's no commonly
 * available means to do so. For example GCC's "#pragma GCC unroll"
 * functionality would be ideal but is only available from GCC 8 onwards. Using
 * -funroll-loops is an option but GCC tends to make poor choices when
 * compiling our string functions. -funroll-all-loops leads to massive code
 * bloat, even if only applied to the string functions.
 */
#define unroll(times, fn, ...) do {				\
	extern void bad_unroll(void)				\
		__compiletime_error("Unsupported unroll");	\
								\
	/*							\
	 * We can't unroll if the number of iterations isn't	\
	 * compile-time constant. Unfortunately GCC versions	\
	 * up until 4.6 tend to miss obvious constants & cause	\
	 * this check to fail, even though they go on to	\
	 * generate reasonable code for the switch statement,	\
	 * so we skip the sanity check for those compilers.	\
	 */							\
	BUILD_BUG_ON((CONFIG_GCC_VERSION >= 40700 ||		\
		      CONFIG_CLANG_VERSION >= 80000) &&		\
		     !__builtin_constant_p(times));		\
								\
	switch (times) {					\
	case 32: fn(__VA_ARGS__); /* fall through */		\
	case 31: fn(__VA_ARGS__); /* fall through */		\
	case 30: fn(__VA_ARGS__); /* fall through */		\
	case 29: fn(__VA_ARGS__); /* fall through */		\
	case 28: fn(__VA_ARGS__); /* fall through */		\
	case 27: fn(__VA_ARGS__); /* fall through */		\
	case 26: fn(__VA_ARGS__); /* fall through */		\
	case 25: fn(__VA_ARGS__); /* fall through */		\
	case 24: fn(__VA_ARGS__); /* fall through */		\
	case 23: fn(__VA_ARGS__); /* fall through */		\
	case 22: fn(__VA_ARGS__); /* fall through */		\
	case 21: fn(__VA_ARGS__); /* fall through */		\
	case 20: fn(__VA_ARGS__); /* fall through */		\
	case 19: fn(__VA_ARGS__); /* fall through */		\
	case 18: fn(__VA_ARGS__); /* fall through */		\
	case 17: fn(__VA_ARGS__); /* fall through */		\
	case 16: fn(__VA_ARGS__); /* fall through */		\
	case 15: fn(__VA_ARGS__); /* fall through */		\
	case 14: fn(__VA_ARGS__); /* fall through */		\
	case 13: fn(__VA_ARGS__); /* fall through */		\
	case 12: fn(__VA_ARGS__); /* fall through */		\
	case 11: fn(__VA_ARGS__); /* fall through */		\
	case 10: fn(__VA_ARGS__); /* fall through */		\
	case 9: fn(__VA_ARGS__); /* fall through */		\
	case 8: fn(__VA_ARGS__); /* fall through */		\
	case 7: fn(__VA_ARGS__); /* fall through */		\
	case 6: fn(__VA_ARGS__); /* fall through */		\
	case 5: fn(__VA_ARGS__); /* fall through */		\
	case 4: fn(__VA_ARGS__); /* fall through */		\
	case 3: fn(__VA_ARGS__); /* fall through */		\
	case 2: fn(__VA_ARGS__); /* fall through */		\
	case 1: fn(__VA_ARGS__); /* fall through */		\
	case 0: break;						\
								\
	default:						\
		/*						\
		 * Either the iteration count is unreasonable	\
		 * or we need to add more cases above.		\
		 */						\
		bad_unroll();					\
		break;						\
	}							\
} while (0)

#endif /* __ASM_UNROLL_H__ */