[PATCH] ARM: NWFPE: avoid compiler-generated __aeabi_uldivmod

From: Nick Desaulniers <ndesaulniers@google.com>
To: Arnd Bergmann <arnd@kernel.org>, Russell King <linux@armlinux.org.uk>
Cc: Tom Rix <trix@redhat.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
	Miguel Ojeda <ojeda@kernel.org>, Ard Biesheuvel <ardb@kernel.org>,
	Gary Guo <gary@garyguo.net>,
	Craig Topper <craig.topper@sifive.com>,
	Philip Reames <listmail@philipreames.com>,
	jh@jhauser.us, Nick Desaulniers <ndesaulniers@google.com>,
	Nathan Chancellor <nathan@kernel.org>
Subject: [PATCH] ARM: NWFPE: avoid compiler-generated __aeabi_uldivmod
Date: Mon, 10 Oct 2022 15:53:42 -0700	[thread overview]
Message-ID: <20221010225342.3903590-1-ndesaulniers@google.com> (raw)
In-Reply-To: <CAKwvOdnQ-tWa0oY0CKAwJD5WQDgVBvxDtf6=E3hS8Ytz5J1OKg@mail.gmail.com>

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(+)

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

