Re: [PATCH] gcc/configure.ac: fix register issue for global_load assembler functions

[Tobias Burnus <tobias@codesourcery.com>]

On 14.06.21 14:36, Julian Brown wrote:
> On Wed, 9 Jun 2021 16:47:21 +0200
> Marcel Vollweiler <marcel@codesourcery.com> wrote:
>> This patch fixes an issue with global_load assembler functions leading
>> to a "invalid operand for instruction" error since in different LLVM
>> versions those functions use either one or two registers.
> LLVM is neither forward- nor backward-compatible with regards to those
> registers then, I guess? That's unfortunate...
The old two-register variant was a bug – which was finally fixed, but it
broke the workaround.
>> In this patch a compatibility check is added to the configure.ac.
> The implementation of the solution looks fine, but I worry it's the
> wrong approach. What would someone packing GCC for a distribution use
> for the configuration setting? It'd mean having a dependency on the
> exact LLVM version for a given offloading-compiler build -- so LLVM
> couldn't be upgraded separately from the offloading compiler. Maybe
> that's OK in practice?

At the end, GCC uses 'as' which is a separate file – which in distros
is usually a symbolic link to the LLVM used in the system.

* Debian uses, https://salsa.debian.org/toolchain-team/gcc

ifneq (,$(filter $(distrelease),buster xenial bionic focal groovy))
   gcn_tools_llvm_version = 9
else ifneq (,$(filter $(distrelease),focal groovy))
   gcn_tools_llvm_version = 11
else
   gcn_tools_llvm_version = tools
endif

which is then used as
   ln -sf /usr/lib/llvm-$(gcn_tools_llvm_version)/bin/llvm-mc bin-gcn/as

Hence, for Debian it encodes the version and we should be fine.
(Not implying that the fixed version dependence is nice.)

* SUSE uses:
ln -s /usr/bin/llvm-mc target-tools/bin/amdgcn-amdhsa-as

I do not quickly see how SUSE solves the LLVM12 version issue.
Richard/Martin and Matthias can answer this better.

I know that for gcn, only, some GCC patches avoid issues like the
.section issue with LLVM 11 but I fail to see how those help with
the assembler problem.

> I wonder if the LLVM assembler has a macro system we could abuse
> instead?

as -defsym=MC_VERSION=`as -v|grep version|sed -e 's/.*version //'` ? ;-)

> Perhaps not. Or another (very ugly) alternative that would work
> with either assembler is giving up and emitting the instruction bit
> patterns directly (as we have done elsewhere for certain "SCC"-setting
> instructions).
Tobias
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