MachineSink: Fix sinking VGPR def out of a divergent loop

This fixes sinking a VGPR def out of a loop past the reconvergence
point at the SI_END_CF. There was a prior fix which introduced
blockPrologueInterferes (D121277) to fix the same basic problem for
the post RA sink. This also had the special case isIgnorableUse case
which was incorrect, because in some contexts the exec use is not
ignorable.

I'm thinking about a new way to represent this which will avoid
needing hasIgnorableUse and isBasicBlockPrologue, which would function
more like the exception handling.

Fixes: SWDEV-407790

https://reviews.llvm.org/D155343

1 files changed, 11 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/MachineSink.cpp b/llvm/lib/CodeGen/MachineSink.cpp
index 3b0166e..8da97dc 100644
--- a/llvm/lib/CodeGen/MachineSink.cpp
+++ b/llvm/lib/CodeGen/MachineSink.cpp
@@ -288,8 +288,7 @@ static bool blockPrologueInterferes(const MachineBasicBlock *BB,
       if (!Reg)
         continue;
       if (MO.isUse()) {
-        if (Reg.isPhysical() &&
-            (TII->isIgnorableUse(MO) || (MRI && MRI->isConstantPhysReg(Reg))))
+        if (Reg.isPhysical() && MRI && MRI->isConstantPhysReg(Reg))
           continue;
         if (PI->modifiesRegister(Reg, TRI))
           return true;
@@ -1007,16 +1006,24 @@ MachineSinking::FindSuccToSinkTo(MachineInstr &MI, MachineBasicBlock *MBB,
   if (MBB == SuccToSinkTo)
     return nullptr;
 
+  if (!SuccToSinkTo)
+    return nullptr;
+
   // It's not safe to sink instructions to EH landing pad. Control flow into
   // landing pad is implicitly defined.
-  if (SuccToSinkTo && SuccToSinkTo->isEHPad())
+  if (SuccToSinkTo->isEHPad())
     return nullptr;
 
   // It ought to be okay to sink instructions into an INLINEASM_BR target, but
   // only if we make sure that MI occurs _before_ an INLINEASM_BR instruction in
   // the source block (which this code does not yet do). So for now, forbid
   // doing so.
-  if (SuccToSinkTo && SuccToSinkTo->isInlineAsmBrIndirectTarget())
+  if (SuccToSinkTo->isInlineAsmBrIndirectTarget())
+    return nullptr;
+
+  MachineBasicBlock::const_iterator InsertPos =
+      SuccToSinkTo->SkipPHIsAndLabels(SuccToSinkTo->begin());
+  if (blockPrologueInterferes(SuccToSinkTo, InsertPos, MI, TRI, TII, MRI))
     return nullptr;
 
   return SuccToSinkTo;