Created attachment 67548 [details]
Test case to reproduce register interference

Problem:

The attached test case reproduces a problem in Gallium NV50 (+NVC0) codegen with register interference tracking for a 64-bit register. The compiler maintains a live register across the execution of a built-in function call which has declared its intention to clobber it.

Explanation:

nv50_ir_ra.cpp:1238 in GCRA::checkInterference uses RegisterSet::occupy to track registers used by interfering nodes. It is possible for the following situation to arise:

  * r0 is set in the RegisterSet
  * RegisterSet::occupy is called to set r0d as well

RegisterSet::occupy will fail to mark r0d as set, by noting the overlap with r0 during the call to BitSet::testRange. This is correct behavior for other users of RegisterSet::occupy.

However, failure is neither checked nor desirable for GCRA::checkInterference. It doesn't matter that two other nodes use overlapping registers - just that the RegisterSet tracks the union of the two as occupied.

Solution:

The attached patch proposes a fix for this. It adds a fourth parameter to RegisterSet::occupy to skip the BitSet::testRange check as the callee desires.

This may not be an optimal solution and I have not confirmed whether similar problems could arise for other users of RegisterSet::occupy. This patch only affects the behavior of the single, known problematic call site.