| Message ID | 20221010225342.3903590-1-ndesaulniers@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | ARM: NWFPE: avoid compiler-generated __aeabi_uldivmod | expand |
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
On Mon, Oct 10, 2022 at 03:53:42PM -0700, 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> I built ARCH=arm allmodconfig + CONFIG_WERROR=n without this patch and saw the link failure then applied it and the error went away. Thanks for all the investigation done into fixing this! I think you put this in the patch tracker already but just for posterity: Tested-by: Nathan Chancellor <nathan@kernel.org> > --- > arch/arm/nwfpe/Makefile | 6 ++++++ > 1 file changed, 6 insertions(+) > > 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-$(CONFIG_FPE_NWFPE_XP) += extended_cpdo.o > + > +# Try really hard to avoid generating calls to __aeabi_uldivmod() from > +# float64_rem() due to loop elision. > +ifdef CONFIG_CC_IS_CLANG > +CFLAGS_softfloat.o += -mllvm -replexitval=never > +endif > -- > 2.38.0.rc2.412.g84df46c1b4-goog >
diff --git a/arch/arm/nwfpe/Makefile b/arch/arm/nwfpe/Makefile index 303400fa2cdf..2aec85ab1e8b 100644 --- a/arch/arm/nwfpe/Makefile +++ b/arch/arm/nwfpe/Makefile @@ -11,3 +11,9 @@ nwfpe-y += fpa11.o fpa11_cpdo.o fpa11_cpdt.o \ entry.o nwfpe-$(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
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> --- arch/arm/nwfpe/Makefile | 6 ++++++ 1 file changed, 6 insertions(+)