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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: auto predicating branches
Date: Tue, 22 Apr 2025 16:45:20 +0000
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On Tue, 22 Apr 2025 5:10:10 +0000, Anton Ertl wrote:

> mitchalsup@aol.com (MitchAlsup1) writes:
--------------
>>
>>My argument is that this is a SW decision (in the compiler) not a
>>HW decision (other than providing the PREDs).
>
> That's a position, not an argument.  Do you have an argument for your
> position?

Architecture defines the primitives, Hardware provides the primitives,
SW decides to use them (or not) on a case-by-case basis.

>>Since PREDs are not
>>predicted (unless you think they are predicted BOTH ways) they do
>>not diminish the performance of the branch predictors.
>
> Nor increase it.

When there is an else-clause, the PRED uses 1 fewer instruction:
the branch that exits the then-clause and jumps over the else-
clause does not exist.

>                   But it sounds like you think that the compiler
> should choose predication when the condition is not particularly
> predictable.  How should the compiler know that?

Compiler is setup to use PRED when the then-clause is 8 or fewer
instructions and when the else clause is 8 or fewer instructions,
otherwise it uses branches.
----------
>
> - anton