Support VCVT for fixed point/float
NEON and VFPv3 have VCVT which can convert between different data types. It's fairly common in codecs to do calculations in fixed point and then convert to floating point using code similar to:
v = (float)i / (1 << constant)
or:
v = i * (1.0f / (1 << constant))
This could be converted into a VCVT.F32.S32.
If y is a power of two then x / y is the same as x * (1/y), so this applies even without -ffast-math. GCC already detects this case and converts the floating point divide into a multiply.
Needs investigation to see what the speed up would be and how often it's used in hot loops. Is used in libav.
Blueprint information
- Status:
- Complete
- Approver:
- Michael Hope
- Priority:
- Medium
- Drafter:
- Michael Hope
- Direction:
- Approved
- Assignee:
- Ramana Radhakrishnan
- Definition:
- Approved
- Series goal:
- Accepted for 4.6
- Implementation:
-
Implemented
- Milestone target:
-
4.6-2012.03
- Started by
- Michael Hope
- Completed by
- Michael Hope
Related branches
Related bugs
Sprints
Whiteboard
Meta:
Headline: Investigate whether conversion from int to float using VCVT can speed up and how often it is used in hot loops
Acceptance: results of the investigation
Scalar patch (vfpv3) submitted here : http://
Generate the equivalent vector (NEON) form: TODO
Summary from the 2011-10-04 performance call:
* Likely to be done through combine
* Likely to be low effort
* Micro improvement is high
* Occurrence is unknown
Approach would be the same for the vector form.
