Messages in this thread | | | Date | Tue, 11 Oct 2022 13:01:22 +0200 | From | "Arnd Bergmann" <> | Subject | Re: [PATCH] ARM: NWFPE: avoid compiler-generated __aeabi_uldivmod |
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On Tue, Oct 11, 2022, at 12:53 AM, Nick Desaulniers wrote: > 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> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Works for me,
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
I would have been fine with disallowing NWFPE for clang, or with adding a barrier in the loop as well, i.e. any approach that doesn't cause invalid behavior or a maintenance burden, given that there is probably nobody that actually needs nwfpe on a clang built kernel.
Arnd
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