About

During 2023 various RISE and non-RISE members worked to improve if-conversion for GCC.  One of the recent discoveries from that work is that it allows if-conversion of a scalar copy of quant_4x4 within the spec2017 benchmark suite.  We have seen this improve the performance of the first (of three) steps in the x264 benchmark by approximately 10%.  The improvement is believed to be primarily due to eliminating a poorly predicted branch in that hot loop.

This work item for LLVM is meant to track a corresponding improvement to LLVM's if-conversion.   While this effort primarily helps the scalar quant_4x4, the belief is that it will help a variety of other codes, just in much smaller ways.

Stakeholders/Partners

RISE:

Ventana: Jeff Law, Mikhail Gudim

External:

Dependencies

Status

Updates

12 Feb 2025 

• Moved to 1H2025

11 Jul 2024 

• Moved to 2H2024

05 Jun 2024 

• Development work is considered done.  Needs to be tested on design to validate improvement though.

25 Apr 2024 

• Failure to convert test into an equality test continues to be a problem
• Craig thinks that's supposed to already work, so clearly needs deeper dive
• Heuristic to avoid if-converting when some operands are invariant seems reasonable, but implementation seems bogus
• Enablement of early if-conversion
• Misha or another Ventana engineer will pick this up.

27 Mar 2024 

• Patch under internal review.  Key issues appear to be:
  • Need to enable early if-conversion by default
  • Wire up additional ways to generate czero
  • Still working through decision whether or not to if-convert

13 Mar 2024 

• Project noted as priority for 20241H