76 lines
2.8 KiB
C
76 lines
2.8 KiB
C
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/* SPDX-License-Identifier: GPL-2.0-only */
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#ifndef __ASM_UNROLL_H__
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#define __ASM_UNROLL_H__
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/*
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* Explicitly unroll a loop, for use in cases where doing so is performance
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* critical.
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*
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* Ideally we'd rely upon the compiler to provide this but there's no commonly
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* available means to do so. For example GCC's "#pragma GCC unroll"
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* functionality would be ideal but is only available from GCC 8 onwards. Using
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* -funroll-loops is an option but GCC tends to make poor choices when
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* compiling our string functions. -funroll-all-loops leads to massive code
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* bloat, even if only applied to the string functions.
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*/
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#define unroll(times, fn, ...) do { \
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extern void bad_unroll(void) \
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__compiletime_error("Unsupported unroll"); \
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\
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/* \
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* We can't unroll if the number of iterations isn't \
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* compile-time constant. Unfortunately clang versions \
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* up until 8.0 tend to miss obvious constants & cause \
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* this check to fail, even though they go on to \
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* generate reasonable code for the switch statement, \
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* so we skip the sanity check for those compilers. \
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*/ \
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BUILD_BUG_ON(!__builtin_constant_p(times)); \
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\
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switch (times) { \
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case 32: fn(__VA_ARGS__); fallthrough; \
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case 31: fn(__VA_ARGS__); fallthrough; \
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case 30: fn(__VA_ARGS__); fallthrough; \
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case 29: fn(__VA_ARGS__); fallthrough; \
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case 28: fn(__VA_ARGS__); fallthrough; \
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case 27: fn(__VA_ARGS__); fallthrough; \
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case 26: fn(__VA_ARGS__); fallthrough; \
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case 25: fn(__VA_ARGS__); fallthrough; \
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case 24: fn(__VA_ARGS__); fallthrough; \
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case 23: fn(__VA_ARGS__); fallthrough; \
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case 22: fn(__VA_ARGS__); fallthrough; \
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case 21: fn(__VA_ARGS__); fallthrough; \
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case 20: fn(__VA_ARGS__); fallthrough; \
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case 19: fn(__VA_ARGS__); fallthrough; \
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case 18: fn(__VA_ARGS__); fallthrough; \
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case 17: fn(__VA_ARGS__); fallthrough; \
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case 16: fn(__VA_ARGS__); fallthrough; \
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case 15: fn(__VA_ARGS__); fallthrough; \
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case 14: fn(__VA_ARGS__); fallthrough; \
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case 13: fn(__VA_ARGS__); fallthrough; \
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case 12: fn(__VA_ARGS__); fallthrough; \
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case 11: fn(__VA_ARGS__); fallthrough; \
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case 10: fn(__VA_ARGS__); fallthrough; \
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case 9: fn(__VA_ARGS__); fallthrough; \
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case 8: fn(__VA_ARGS__); fallthrough; \
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case 7: fn(__VA_ARGS__); fallthrough; \
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case 6: fn(__VA_ARGS__); fallthrough; \
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case 5: fn(__VA_ARGS__); fallthrough; \
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case 4: fn(__VA_ARGS__); fallthrough; \
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case 3: fn(__VA_ARGS__); fallthrough; \
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case 2: fn(__VA_ARGS__); fallthrough; \
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case 1: fn(__VA_ARGS__); fallthrough; \
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case 0: break; \
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\
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default: \
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/* \
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* Either the iteration count is unreasonable \
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* or we need to add more cases above. \
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*/ \
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bad_unroll(); \
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break; \
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} \
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} while (0)
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#endif /* __ASM_UNROLL_H__ */
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