Re: [PATCH v2 02/10] target/hexagon: import README for idef-parser

[Alessandro Di Federico via <qemu-devel@nongnu.org>]

On Thu, 25 Feb 2021 12:20:53 -0800
Richard Henderson <richard.henderson@linaro.org> wrote:
> This is odd, as is the description of why.  Yes, if RdV is read
> without initialization, TCG middle-end will abort (at least with
> --enable-debug-tcg enabling the assertions).  But you've just said
> that "no reading" was found.

When we say that no reading was found, we mean that it was not
initialized by the caller. The term "reading" is a leftover from how
the parser input was organized in the first iterations of the base
patchset.

I'll rephrase that.

> So why did you perform this dummy initialization, which will be
> eliminated later?

The initialization is redundant in this specific example, but, in
general, non-initialized values are assumed to be zero-initialized.
For instance when you're writing a 64-bit integer piecewise, by OR-ing
two 32-bit integers, it matters.

In short: useless in this case (but DCE'd by the mid-end), important in
general.
 
> So, I take it from this that you're emitting tcg directly from within
> the parser, and not generating any kind of abstract syntax tree?

Yes. There a few spots where an AST would have been beneficial, but
overall we deem it would increase the complexity of the parser with
limited return.

-- 
Alessandro Di Federico
rev.ng