Proposal: Add Autocompress Control

[lenary]

These options allow users better control of when the assembler should turn specific instructions into their smaller equivalents, without having to change the enabled architectures.

A prototype LLVM implementation is available here: llvm/llvm-project#122483

[topperc]

I think the "when" in "instead when assembly writers" is unnecessary.

[psherman42]

Do we really need another option, when there are already existing options and best practices?

.option push
.option norvc
...
.option pop

[lenary]

Do we really need another option, when there are already existing options and best practices?

I put more motivation in the LLVM message, for which I apologise, I was writing both messages at the same time. From that motivation:

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This will become more useful as the following things happen:

• .option norvc is deprecated/removed (which is sometimes used for this purpose).
• Extensions are added where the destination instruction cannot be disabled separately to the source instruction, either because the destination is in the base architecture, or because it is in the same extension as the source.
• Extensions wider than 32-bits are added, which make CompressPats [the feature used to implement this in LLVM] more complex to use intuitively, especially if the destination is a 32-bit instruction.

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This document says the following about .option norvc, which suggests it is not a best practice:

This option will be deprecated soon after .option arch has been widely implemented on main stream open source toolchains.

Another problem with .option (no)rvc is that it is stated to only disable/enable the C extension, and how it interacts with e.g. Zca/Zcf/Zcd is underspecified, not very obvious, and complex.

In short, yes I do think we need an option for controlling this feature.

I'll note that the proposed LLVM implementation correctly works with .option push and .option pop, which is maybe not obvious from the diff.