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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>
20 lines
521 B
Makefile
20 lines
521 B
Makefile
# SPDX-License-Identifier: GPL-2.0
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#
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# Copyright (C) 1998, 1999, 2001 Philip Blundell
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#
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obj-$(CONFIG_FPE_NWFPE) += nwfpe.o
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nwfpe-y += fpa11.o fpa11_cpdo.o fpa11_cpdt.o \
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fpa11_cprt.o fpmodule.o fpopcode.o \
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softfloat.o single_cpdo.o double_cpdo.o \
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entry.o
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nwfpe-$(CONFIG_FPE_NWFPE_XP) += extended_cpdo.o
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# Try really hard to avoid generating calls to __aeabi_uldivmod() from
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# float64_rem() due to loop elision.
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ifdef CONFIG_CC_IS_CLANG
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CFLAGS_softfloat.o += -mllvm -replexitval=never
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endif
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