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Re: R: [RFC PATCH v2] target/ppc: Enable hardfloat for PPC

From: Alex Bennée
Subject: Re: R: [RFC PATCH v2] target/ppc: Enable hardfloat for PPC
Date: Wed, 26 Feb 2020 14:28:45 +0000
User-agent: mu4e 1.3.8; emacs 27.0.60

Dino Papararo <address@hidden> writes:

> Please let's go with hardfloat pps support, it's really a good feature to 
> implement.
> Even if in a first step it could lead to inaccuracy results, later it
> could solved with other patches.

That's the wrong way around. We have regression tests for a reason. I'll
happily accept patches to turn on hardfloat for PPC if:

 a) they don't cause regressions in our fairly extensive floating point
 b) the PPC maintainers are happy with the new performance profile

The way forward would be to:

 1. patch to drop #if defined(TARGET_PPC) || defined(__FAST_MATH__)
 2. audit target/ppc/fpu_helper.c w.r.t chip manual and fix any unneeded
 splatting of flags (if any)
 3. measure the before/after performance effect and decide if on balance
 it's worth keeping

> I think it's important for qemu to as global as possible and don't
> target only recent hardware.

Are you referring to guests or hosts? For guests we will always favour
accuracy of speed of emulation. For hosts we need to have IEEE compliant
FPU HW to even stand a chance of using hardfloat.

Alex Bennée

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