On 04/02/16 17:12, Andrew Pinski wrote:
On Thu, Feb 4, 2016 at 5:50 AM, Kyrill Tkachov
<kyrylo.tkac...@foss.arm.com> wrote:
Hi all,
As part of the target attributes and pragmas support for GCC 6 I changed the
aarch64 port
to emit a .arch assembly directive for each function that describes the
architectural features
used by that function. This is a change from GCC 5 behaviour where we
output a single .arch directive
at the beginning of the assembly file corresponding to architectural
features given on the command line.
We need that change to accommodate use cases where a user tags a single
function with a target attribute
that changes the arch features from those given on the command line and then
proceeds to use an inline
assembly instruction that needs those new features. We need to emit the new
architecture state for that
function as a .arch directive to prevent the assembler from rejecting the
assembly.
Unfortunately, the new behaviour has caused some headaches when building the
Linux kernel.
There, they have a header called lse.h that is used to access some
armv8.1-a-specific inline assembly
(using the "lse" architecture feature in particular)
This header has the line:
__asm__(".arch_extension lse");
to tell the assembler to accept LSE instructions.
This header is included in various .c files so the .arch_extension appears
at the beginning of the
assembly file.
But since we now emit a .arch directive for every function, we effectively
reset the architectural state
on each function, rendering that .arch_extension ineffective.
GCC is arguably right in emitting the .arch directive on each function, but
I'd like to make life easier
for the kernel folk so this patch helps with that.
I had suggested a change to the Linux kernel already to fix this issue:
http://lists.infradead.org/pipermail/linux-arm-kernel/2015-December/395191.html
Indeed, but I don't see this patch in the 4.3 or 4.4 Linux kernel
releases that I tried, so we'd still want GCC 6 to be able to compile those.
Kyrill
Thanks,
Andrew Pinski
With this patch we go back to emitting the .arch directive at the top of the
assembly file, same in GCC 5.
We will still emit per-function .arch directives but only if they differ
from the last .arch directive
that we emitted i.e. when the architecture features were changed due to a
target attribute or pragma.
For code that doesn't make use of target attributes or pragmas the behaviour
of GCC 5 is thus preserved
and we still get the correct .arch updates when needed.
The TARGET_ASM_FILE_START hook implementation is brought back after I
deleted it during the target
attributes work last August and is used again to output the .arch directive
at the top of the file.
We keep track of the architecture and the architecture features we last
output in the asm directive
in a new static variable (yuck, but unavoidable IMO) and output the new
.arch directive only if
what we want to output is different from the previously output directive.
This still allows the kernel to do naughty things with .arch_extension
directives in header files,
but this usecase is one where they really know what they're doing and don't
want the assembler
to get in their way.
Bootstrapped and tested on aarch64-none-linux-gnu.
With this patch I managed to build a recent allyesconfig Linux kernel where
before the build would fail
when assembling the LSE instructions.
Ok for trunk?
Thanks,
Kyrill
2016-02-04 Kyrylo Tkachov <kyrylo.tkac...@arm.com>
* config/aarch64/aarch64.c (struct aarch64_output_asm_info):
New struct definition.
(aarch64_previous_asm_output): New variable.
(aarch64_declare_function_name): Only output .arch assembler
directive if it will be different from the previously output
directive.
(aarch64_start_file): New function.
(TARGET_ASM_FILE_START): Define.
2016-02-04 Kyrylo Tkachov <kyrylo.tkac...@arm.com>
* gcc.target/aarch64/assembler_arch_1.c: Add -dA to dg-options.
Delete unneeded -save-temps.
* gcc.target/aarch64/assembler_arch_7.c: Likewise.
* gcc.target/aarch64/target_attr_15.c: Scan assembly for
.arch armv8-a\n.
* gcc.target/aarch64/assembler_arch_1.c: New test.
