[PATCH] D63707: [AArch64] Define ETE and TRBE system registers

Mon Jul 1 04:03:11 PDT 2019

chill added a comment.

In D63707#1564007 <https://reviews.llvm.org/D63707#1564007>, @ostannard wrote:

> > ETE shares most of the system registers with ETM and these were historically unconditionally enabled in LLVM.

> I don't think this matters here - existing registers can stay as they are, and the new subtarget features should just cover new registers.

>> It would be inconsistent to add a command line option, which enables just the few extra ETE registers
> 
> I don't see why this would be inconsistent, we now add subtarget features for every other new optional architectural feature. Maybe we should add a feature for ETM if it is optional, but that doesn't affect this patch.

ETE is not an extension over or a next version of  ETM - it's a completely standalone extension that happens to share with ETM many register names and encodings, but not necessarily register semantics.
I don't think it's logical for any extension to have a mix of conditional/unconditional availability of registers -  either all registers should be unconditionally available,
or all registers should be  behind a target feature and a command line option.  But making all of the ETE registers conditional on a target feature would affect that set of ETM registers that
have the same name and encoding, breaking compatibility.

>> or it would break existing builds if we were to explicitly enable ETM registers.
> 
> Do you mean if we were to make ETM off by default? Agreed, we shouldn't do that.

Yes, exactly, I meant "if we were to require explicitly enabling ETM registers".

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https://reviews.llvm.org/D63707