From: Nick Desaulniers ndesaulniers@google.com
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 --- arch/arm/nwfpe/Makefile | 6 ++++++ 1 file changed, 6 insertions(+)
--- a/arch/arm/nwfpe/Makefile +++ b/arch/arm/nwfpe/Makefile @@ -10,3 +10,9 @@ nwfpe-y += fpa11.o fpa11_cpdo.o fpa11 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