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Message-Id: <20190126040111.9013-1-natechancellor@gmail.com>
Date: Fri, 25 Jan 2019 21:01:13 -0700
From: Nathan Chancellor <natechancellor@...il.com>
To: Russell King <linux@...linux.org.uk>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Jonathan Corbet <corbet@....net>
Cc: linux-doc@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org, Tri Vo <trong@...roid.com>,
Stefan Agner <stefan@...er.ch>,
Nicolas Pitre <nicolas.pitre@...aro.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Nathan Chancellor <natechancellor@...il.com>,
Nicolas Pitre <nico@...aro.org>
Subject: [PATCH RESEND] ARM: Ensure that NEON code always compiles with Clang
While building arm32 allyesconfig, I ran into the following errors:
arch/arm/lib/xor-neon.c:17:2: error: You should compile this file with
'-mfloat-abi=softfp -mfpu=neon'
In file included from lib/raid6/neon1.c:27:
/home/nathan/cbl/prebuilt/lib/clang/8.0.0/include/arm_neon.h:28:2:
error: "NEON support not enabled"
Building V=1 showed NEON_FLAGS getting passed along to Clang but
__ARM_NEON__ was not getting defined. Ultimately, it boils down to Clang
only defining __ARM_NEON__ when targeting armv7, rather than armv6k,
which is the '-march' value for allyesconfig.
>>From lib/Basic/Targets/ARM.cpp in the Clang source:
// This only gets set when Neon instructions are actually available, unlike
// the VFP define, hence the soft float and arch check. This is subtly
// different from gcc, we follow the intent which was that it should be set
// when Neon instructions are actually available.
if ((FPU & NeonFPU) && !SoftFloat && ArchVersion >= 7) {
Builder.defineMacro("__ARM_NEON", "1");
Builder.defineMacro("__ARM_NEON__");
// current AArch32 NEON implementations do not support double-precision
// floating-point even when it is present in VFP.
Builder.defineMacro("__ARM_NEON_FP",
"0x" + Twine::utohexstr(HW_FP & ~HW_FP_DP));
}
Ard Biesheuvel recommended explicitly adding '-march=armv7-a' at the
beginning of the NEON_FLAGS definitions so that __ARM_NEON__ always gets
definined by Clang. This doesn't functionally change anything because
that code will only run where NEON is supported, which is implicitly
armv7.
Link: https://github.com/ClangBuiltLinux/linux/issues/287
Suggested-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
Acked-by: Nicolas Pitre <nico@...aro.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
Resending with Nicolas's ack and Nick's review, to give others a chance
to pitch in with review/testing before submitting it to the patch
system (specifically Stefan, sorry I should have included you in the
previous posting).
With this patch and:
* A tip of tree LLVM build (I used apt.llvm.org)
* Clang GCOV support: https://lore.kernel.org/lkml/20190122233749.42220-1-trong@android.com/
* Two minor hacks for unrelated issues that are still being worked through:
* https://raw.githubusercontent.com/nathanchance/patches/c313b2fa0efb/linux/build-hax/0003-DO-NOT-UPSTREAM-ARM-Don-t-select-HAVE_FUNCTION_TRACE.patch (see https://github.com/ClangBuiltLinux/linux/issues/35)
* https://gist.githubusercontent.com/nathanchance/b2f5a4015abade1a41e78d5fc3235c5b/raw/744321882ab05511331f26896bad7c9f0056a6a5/gistfile1.txt (see https://github.com/ClangBuiltLinux/linux/issues/325)
I can build and link a little endian allyesconfig ARM kernel. This
patch alone works with GCC 8.2.0 and binutils 2.31.1.
Documentation/arm/kernel_mode_neon.txt | 4 ++--
arch/arm/lib/Makefile | 2 +-
arch/arm/lib/xor-neon.c | 2 +-
lib/raid6/Makefile | 2 +-
4 files changed, 5 insertions(+), 5 deletions(-)
diff --git a/Documentation/arm/kernel_mode_neon.txt b/Documentation/arm/kernel_mode_neon.txt
index 525452726d31..b9e060c5b61e 100644
--- a/Documentation/arm/kernel_mode_neon.txt
+++ b/Documentation/arm/kernel_mode_neon.txt
@@ -6,7 +6,7 @@ TL;DR summary
* Use only NEON instructions, or VFP instructions that don't rely on support
code
* Isolate your NEON code in a separate compilation unit, and compile it with
- '-mfpu=neon -mfloat-abi=softfp'
+ '-march=armv7-a -mfpu=neon -mfloat-abi=softfp'
* Put kernel_neon_begin() and kernel_neon_end() calls around the calls into your
NEON code
* Don't sleep in your NEON code, and be aware that it will be executed with
@@ -87,7 +87,7 @@ instructions appearing in unexpected places if no special care is taken.
Therefore, the recommended and only supported way of using NEON/VFP in the
kernel is by adhering to the following rules:
* isolate the NEON code in a separate compilation unit and compile it with
- '-mfpu=neon -mfloat-abi=softfp';
+ '-march=armv7-a -mfpu=neon -mfloat-abi=softfp';
* issue the calls to kernel_neon_begin(), kernel_neon_end() as well as the calls
into the unit containing the NEON code from a compilation unit which is *not*
built with the GCC flag '-mfpu=neon' set.
diff --git a/arch/arm/lib/Makefile b/arch/arm/lib/Makefile
index ad25fd1872c7..0bff0176db2c 100644
--- a/arch/arm/lib/Makefile
+++ b/arch/arm/lib/Makefile
@@ -39,7 +39,7 @@ $(obj)/csumpartialcopy.o: $(obj)/csumpartialcopygeneric.S
$(obj)/csumpartialcopyuser.o: $(obj)/csumpartialcopygeneric.S
ifeq ($(CONFIG_KERNEL_MODE_NEON),y)
- NEON_FLAGS := -mfloat-abi=softfp -mfpu=neon
+ NEON_FLAGS := -march=armv7-a -mfloat-abi=softfp -mfpu=neon
CFLAGS_xor-neon.o += $(NEON_FLAGS)
obj-$(CONFIG_XOR_BLOCKS) += xor-neon.o
endif
diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c
index 2c40aeab3eaa..c691b901092f 100644
--- a/arch/arm/lib/xor-neon.c
+++ b/arch/arm/lib/xor-neon.c
@@ -14,7 +14,7 @@
MODULE_LICENSE("GPL");
#ifndef __ARM_NEON__
-#error You should compile this file with '-mfloat-abi=softfp -mfpu=neon'
+#error You should compile this file with '-march=armv7-a -mfloat-abi=softfp -mfpu=neon'
#endif
/*
diff --git a/lib/raid6/Makefile b/lib/raid6/Makefile
index 4e90d443d1b0..e723eacf7868 100644
--- a/lib/raid6/Makefile
+++ b/lib/raid6/Makefile
@@ -39,7 +39,7 @@ endif
ifeq ($(CONFIG_KERNEL_MODE_NEON),y)
NEON_FLAGS := -ffreestanding
ifeq ($(ARCH),arm)
-NEON_FLAGS += -mfloat-abi=softfp -mfpu=neon
+NEON_FLAGS += -march=armv7-a -mfloat-abi=softfp -mfpu=neon
endif
CFLAGS_recov_neon_inner.o += $(NEON_FLAGS)
ifeq ($(ARCH),arm64)
--
2.20.1
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