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

[Julian Brown <julian@codesourcery.com>]

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...

> 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?

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

Julian