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Bug 1191 - Clang/ARM transforms VMLA into VMUL+VADD under our feet!
Summary: Clang/ARM transforms VMLA into VMUL+VADD under our feet!
Alias: None
Product: Eigen
Classification: Unclassified
Component: Core - vectorization (show other bugs)
Version: 3.3 (current stable)
Hardware: ARM - NEON Android
: Normal Unknown
Assignee: Nobody
Depends on:
Reported: 2016-04-04 20:38 UTC by Benoit Jacob
Modified: 2016-04-04 20:47 UTC (History)
4 users (show)


Description Benoit Jacob 2016-04-04 20:38:11 UTC
See e.g.

For us this is a disaster, since this means that we can't trust EIGEN_HAS_SINGLE_INSTRUCTION_MADD. So we end up mis-shooting the number of registers, and spilling!

The work-around for this is going to be to reimplement the pmadd in inline assembly.
Comment 1 Benoit Jacob 2016-04-04 20:39:52 UTC
 * this affects all known versions of Clang/ARM, but not Aarch64.
 * this depends on -mcpu= settings. Affected settings include -mcpu=cortex-a8 and -mcpu=cortex-a7. Since the former is the default on -march=armv7-a, that is a very common case.

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