[MachineSink] Allow sinking of constant or ignorable physreg uses

For AMDGPU, any use of the physical register EXEC prevents sinking even if it is not a real physical register read. Add check to see if a physical
register use can be ignored for sinking.

Also perform same constant and ignorable physical register check when considering sinking in loops.

https://reviews.llvm.org/D116053

1 files changed, 8 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/MachineSink.cpp b/llvm/lib/CodeGen/MachineSink.cpp
index 54c4786..0dbbc21 100644
--- a/llvm/lib/CodeGen/MachineSink.cpp
+++ b/llvm/lib/CodeGen/MachineSink.cpp
@@ -796,9 +796,14 @@ bool MachineSinking::isProfitableToSinkTo(Register Reg, MachineInstr &MI,
     if (Reg == 0)
       continue;
 
-    // Don't handle physical register.
-    if (Register::isPhysicalRegister(Reg))
+    if (Register::isPhysicalRegister(Reg)) {
+      if (MO.isUse() &&
+          (MRI->isConstantPhysReg(Reg) || TII->isIgnorableUse(MO)))
+        continue;
+
+      // Don't handle non-constant and non-ignorable physical register.
       return false;
+    }
 
     // Users for the defs are all dominated by SuccToSinkTo.
     if (MO.isDef()) {
@@ -898,7 +903,7 @@ MachineSinking::FindSuccToSinkTo(MachineInstr &MI, MachineBasicBlock *MBB,
         // If the physreg has no defs anywhere, it's just an ambient register
         // and we can freely move its uses. Alternatively, if it's allocatable,
         // it could get allocated to something with a def during allocation.
-        if (!MRI->isConstantPhysReg(Reg))
+        if (!MRI->isConstantPhysReg(Reg) && !TII->isIgnorableUse(MO))
           return nullptr;
       } else if (!MO.isDead()) {
         // A def that isn't dead. We can't move it.