6ec4476ac8
I realize that we fairly recently raised it to 4.8, but the fact is, 4.9
is a much better minimum version to target.
We have a number of workarounds for actual bugs in pre-4.9 gcc versions
(including things like internal compiler errors on ARM), but we also
have some syntactic workarounds for lacking features.
In particular, raising the minimum to 4.9 means that we can now just
assume _Generic() exists, which is likely the much better replacement
for a lot of very convoluted built-time magic with conditionals on
sizeof and/or __builtin_choose_expr() with same_type() etc.
Using _Generic also means that you will need to have a very recent
version of 'sparse', but thats easy to build yourself, and much less of
a hassle than some old gcc version can be.
The latest (in a long string) of reasons for minimum compiler version
upgrades was commit 5435f73d5c ("efi/x86: Fix build with gcc 4").
Ard points out that RHEL 7 uses gcc-4.8, but the people who stay back on
old RHEL versions persumably also don't build their own kernels anyway.
And maybe they should cross-built or just have a little side affair with
a newer compiler?
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
77 lines
3.0 KiB
C
77 lines
3.0 KiB
C
/* 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((CONFIG_CLANG_VERSION >= 80000) && \
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!__builtin_constant_p(times)); \
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\
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switch (times) { \
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case 32: fn(__VA_ARGS__); /* fall through */ \
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case 31: fn(__VA_ARGS__); /* fall through */ \
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case 30: fn(__VA_ARGS__); /* fall through */ \
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case 29: fn(__VA_ARGS__); /* fall through */ \
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case 28: fn(__VA_ARGS__); /* fall through */ \
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case 27: fn(__VA_ARGS__); /* fall through */ \
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case 26: fn(__VA_ARGS__); /* fall through */ \
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case 25: fn(__VA_ARGS__); /* fall through */ \
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case 24: fn(__VA_ARGS__); /* fall through */ \
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case 23: fn(__VA_ARGS__); /* fall through */ \
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case 22: fn(__VA_ARGS__); /* fall through */ \
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case 21: fn(__VA_ARGS__); /* fall through */ \
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case 20: fn(__VA_ARGS__); /* fall through */ \
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case 19: fn(__VA_ARGS__); /* fall through */ \
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case 18: fn(__VA_ARGS__); /* fall through */ \
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case 17: fn(__VA_ARGS__); /* fall through */ \
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case 16: fn(__VA_ARGS__); /* fall through */ \
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case 15: fn(__VA_ARGS__); /* fall through */ \
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case 14: fn(__VA_ARGS__); /* fall through */ \
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case 13: fn(__VA_ARGS__); /* fall through */ \
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case 12: fn(__VA_ARGS__); /* fall through */ \
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case 11: fn(__VA_ARGS__); /* fall through */ \
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case 10: fn(__VA_ARGS__); /* fall through */ \
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case 9: fn(__VA_ARGS__); /* fall through */ \
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case 8: fn(__VA_ARGS__); /* fall through */ \
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case 7: fn(__VA_ARGS__); /* fall through */ \
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case 6: fn(__VA_ARGS__); /* fall through */ \
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case 5: fn(__VA_ARGS__); /* fall through */ \
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case 4: fn(__VA_ARGS__); /* fall through */ \
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case 3: fn(__VA_ARGS__); /* fall through */ \
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case 2: fn(__VA_ARGS__); /* fall through */ \
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case 1: fn(__VA_ARGS__); /* fall through */ \
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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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