diff options
| author | Nick Desaulniers | 2022-10-11 20:00:12 +0100 |
|---|---|---|
| committer | Greg Kroah-Hartman | 2023-01-07 11:11:49 +0100 |
| commit | 94d23f8b78400a7f04da6d77abc294dd56daaea0 (patch) | |
| tree | 711cc295cea9c9612170b0eed3e79207ed4a9b4b /arch | |
| parent | 2833221e0b0f7a7b3e82e6a68f0407f1808ffc44 (diff) | |
ARM: 9256/1: NWFPE: avoid compiler-generated __aeabi_uldivmod
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>
Diffstat (limited to 'arch')
| -rw-r--r-- | arch/arm/nwfpe/Makefile | 6 |
1 files changed, 6 insertions, 0 deletions
diff --git a/arch/arm/nwfpe/Makefile b/arch/arm/nwfpe/Makefile index 303400fa2cdf..2aec85ab1e8b 100644 --- a/arch/arm/nwfpe/Makefile +++ b/arch/arm/nwfpe/Makefile @@ -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 |