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Varying unit (V) reported by Offline Compiler

Tzuchien Chiu

I'd like to know what it means when a fragment shader is bound by Varying unity (V), in our case.

According to: https://developer.arm.com/documentation/101863/7-4/Mali-GPU-pipelines/Mali-Bifrost-architecture

The varying pipeline is a dedicated pipeline which implements the varying interpolator.

Does it mean that the it takes a lot of cycles just interpolating the varyings than ALU operations, and reducing the amount of varyings could potentially reduce the fragment shader cycles ?

For example:

Mali Offline Compiler v7.4.0 (Build 330167)
Copyright 2007-2021 Arm Limited, all rights reserved

Configuration
=============

Hardware: Mali-G71 r0p1
Architecture: Bifrost
Driver: r32p0-00rel0
Shader type: OpenGL ES Fragment

Main shader
===========

Work registers: 24
Uniform registers: 12
Stack spilling: false
16-bit arithmetic: 60%

A LS V T Bound
Total instruction cycles: 1.42 0.00 3.50 2.00 V
Shortest path cycles: 1.42 0.00 3.50 2.00 V
Longest path cycles: 1.42 0.00 3.50 2.00 V

A = Arithmetic, LS = Load/Store, V = Varying, T = Texture

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  • 0 in reply to Peter Harris

    This makes sense and surprises me at the same time. Thank you a lot because I would never figure it out on my own. 
    - Is there any counter in Streamline which help us detect this kind of promotion
    - Is there any other precision surprises we should expect from the compiler ?
    - Is there anyway to defeat this optimization ? (say, the texture is very small)

Reply
  • 0 in reply to Peter Harris

    This makes sense and surprises me at the same time. Thank you a lot because I would never figure it out on my own. 
    - Is there any counter in Streamline which help us detect this kind of promotion
    - Is there any other precision surprises we should expect from the compiler ?
    - Is there anyway to defeat this optimization ? (say, the texture is very small)

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