Some PINGs

Roger Sayle roger@nextmovesoftware.com
Sun Nov 7 14:07:09 GMT 2021


>On 11/6/2021 4:20 PM, Roger Sayle wrote:
>> Simplify paradoxical subreg extensions of TRUNCATE 
>> https://gcc.gnu.org/pipermail/gcc-patches/2021-September/578848.html
> So the discussion seemed to end with a recommendation to try and address this earlier in the call chain rather than in simplify_rtx.

I take full responsibility that my poor choice of wording in describing this patch
may be confusing to those not familiar with this style of describing middle-end
transformations in GCC.  Take the opening sentence "This patch simplifies the
RTX (subreg:HI (truncate:QI (reg:SI))) as (truncate:HI (reg:SI))", taken literally
someone may look-up the RTL documentation and observe that SUBREG
may never be applied to truncate hence the above is invalid, and therefore
this transformation could never fire.  The wording is perhaps unfortunate; the
abstract RTL above is never actually created/allocated, instead we're describing the
transformation/context, i.e. the behaviour of the call to gen_lowpart when passed
a TRUCATE rtx as an operand.  Hence, this patch is already "earlier in the call chain"
and where it should be.  This is no different to how GCC's match describes tree
expressions to be folded, but that never physically exist in GIMPLE, where the
single-static assignment form guarantees no operator may have another as an
operand.

If someone could review the code change itself, and not misinterpret the patch 
description that would be much appreciated.

Thanks in advance,
Roger
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