[MachineLICM] Hoist TOC-based address instructions

Add condition for MachineLICM to safely hoist instructions that utilize
non constant registers that are reserved.

On PPC, global variable access is done through the table of contents (TOC)
which is always in register X2.  The ABI reserves this register in any
functions that have calls or access global variables.

A call through a function pointer involves saving, changing and restoring
this register around the call and thus MachineLICM does not consider it to
be invariant. We can however guarantee the register is preserved across the
call and thus is invariant.

Differential Revision: https://reviews.llvm.org/D33562

llvm-svn: 305490

1 files changed, 5 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/MachineLICM.cpp b/llvm/lib/CodeGen/MachineLICM.cpp
index 52d5819f8db..c7113f1fdc4 100644
--- a/llvm/lib/CodeGen/MachineLICM.cpp
+++ b/llvm/lib/CodeGen/MachineLICM.cpp
@@ -895,8 +895,11 @@ bool MachineLICM::IsLoopInvariantInst(MachineInstr &I) {
         // 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))
-          return false;
+        // However, if the physreg is known to always be caller saved/restored
+        // then this use is safe to hoist.
+        if (!MRI->isConstantPhysReg(Reg) &&
+            !(TRI->isCallerPreservedPhysReg(Reg, *I.getParent()->getParent())))
+            return false;
         // Otherwise it's safe to move.
         continue;
       } else if (!MO.isDead()) {