This is a note to let you know that I've just added the patch titled
ARM: 9256/1: NWFPE: avoid compiler-generated __aeabi_uldivmod
to the 6.0-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
arm-9256-1-nwfpe-avoid-compiler-generated-__aeabi_uldivmod.patch
and it can be found in the queue-6.0 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 3220022038b9a3845eea762af85f1c5694b9f861 Mon Sep 17 00:00:00 2001
From: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Date: Tue, 11 Oct 2022 20:00:12 +0100
Subject: ARM: 9256/1: NWFPE: avoid compiler-generated __aeabi_uldivmod
From: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
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@xxxxxxxxxx>
Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Russell King (Oracle) <rmk+kernel@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/arm/nwfpe/Makefile | 6 ++++++
1 file changed, 6 insertions(+)
--- a/arch/arm/nwfpe/Makefile
+++ b/arch/arm/nwfpe/Makefile
@@ -11,3 +11,9 @@ 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
Patches currently in stable-queue which might be from ndesaulniers@xxxxxxxxxx are
queue-6.0/arm-9256-1-nwfpe-avoid-compiler-generated-__aeabi_uldivmod.patch
