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ARM: 9256/1: NWFPE: avoid compiler-generated __aeabi_uldivmod

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commit 3220022038b9a3845eea762af85f1c5694b9f861 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Nick Desaulniers 2022-10-11 20:00:12 +01:00 committed by Greg Kroah-Hartman
parent c42cb66a89
commit 50c2379f87
1 changed files with 6 additions and 0 deletions
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6
arch/arm/nwfpe/Makefile
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@ -11,3 +11,9 @@ nwfpe-y += fpa11.o fpa11_cpdo.o fpa11_cpdt.o \
entry.o
nwfpe-$(CONFIG_FPE_NWFPE_XP) += extended_cpdo.o
# Try really hard to avoid generating calls to __aeabi_uldivmod() from
# float64_rem() due to loop elision.
ifdef CONFIG_CC_IS_CLANG
CFLAGS_softfloat.o += -mllvm -replexitval=never
endif