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clang-15's ability to elide loops completely became more aggressive when it can deduce how a variable is being updated in a loop. Counting down one variable by an increment of another can be replaced by a modulo operation. For 64b variables on 32b ARM EABI targets, this can result in the compiler generating calls to __aeabi_uldivmod, which it does for a do while loop in float64_rem(). For the kernel, we'd generally prefer that developers not open code 64b division via binary / operators and instead use the more explicit helpers from div64.h. On arm-linux-gnuabi targets, failure to do so can result in linkage failures due to undefined references to __aeabi_uldivmod(). While developers can avoid open coding divisions on 64b variables, the compiler doesn't know that the Linux kernel has a partial implementation of a compiler runtime (--rtlib) to enforce this convention. It's also undecidable for the compiler whether the code in question would be faster to execute the loop vs elide it and do the 64b division. While I actively avoid using the internal -mllvm command line flags, I think we get better code than using barrier() here, which will force reloads+spills in the loop for all toolchains. Link: https://github.com/ClangBuiltLinux/linux/issues/1666 Reported-by: Nathan Chancellor <nathan@kernel.org> Reviewed-by: Arnd Bergmann <arnd@arndb.de> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> Tested-by: Nathan Chancellor <nathan@kernel.org> Cc: stable@vger.kernel.org Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk> |
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| .. | ||
| ARM-gcc.h | ||
| ChangeLog | ||
| double_cpdo.c | ||
| entry.S | ||
| extended_cpdo.c | ||
| fpa11.c | ||
| fpa11.h | ||
| fpa11.inl | ||
| fpa11_cpdo.c | ||
| fpa11_cpdt.c | ||
| fpa11_cprt.c | ||
| fpmodule.c | ||
| fpmodule.h | ||
| fpmodule.inl | ||
| fpopcode.c | ||
| fpopcode.h | ||
| fpsr.h | ||
| Makefile | ||
| milieu.h | ||
| single_cpdo.c | ||
| softfloat-macros | ||
| softfloat-specialize | ||
| softfloat.c | ||
| softfloat.h | ||