[LLVMdev] Predication bug in AggressiveAntiDepBreaker?

Verena Beckham verena at codeplay.com
Mon Jul 22 09:18:53 PDT 2013


I wondered whether the AggressiveAntiDepBreaker can properly handle 
predicated instructions.

At the end of PrescanInstruction the "DefIndices" array is updated with 
the destination register without checking whether the instruction is 
predicated. That shortens the live range: Later on, in HandleLastUse we 
check whether the register IsLive, which considers only "KillIndices" 
and "DefIndices", and therefore returns False for the interval between 
the predicated instruction and any non-predicated read before it. So 
that read is considered the last use.
In my example this leads to a register not being fully renamed everywhere.

I don't think a predicated write should count as a define.

The attached patch fixes this test case.

Or am I missing something target dependent here?

Verena Beckham

Vice President Engineering

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-------------- next part --------------
Index: AggressiveAntiDepBreaker.cpp
--- AggressiveAntiDepBreaker.cpp	(revision 186828)
+++ AggressiveAntiDepBreaker.cpp	(working copy)
@@ -399,7 +399,7 @@
     unsigned Reg = MO.getReg();
     if (Reg == 0) continue;
     // Ignore KILLs and passthru registers for liveness...
-    if (MI->isKill() || (PassthruRegs.count(Reg) != 0))
+    if (MI->isKill() || (PassthruRegs.count(Reg) != 0) || TII->isPredicated(MI))
     // Update def for Reg and aliases.

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