[LICM] Hoist assumes out of loops

If we have an assume which is known to execute and whose operand is invariant, we can lift that into the pre-header. So long as we don't change which paths the assume executes on, this is a legal transformation. It's likely to be a useful canonicalization as other transforms only look for dominating assumes.

Differential Revision: https://reviews.llvm.org/D50364

llvm-svn: 339481

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index 4150d9fece1..b99c6d3d4ea 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -658,6 +658,15 @@ bool llvm::canSinkOrHoistInst(Instruction &I, AAResults *AA, DominatorTree *DT,
     if (CI->mayThrow())
       return false;
 
+    if (Function *F = CI->getCalledFunction())
+        switch (F->getIntrinsicID()) {
+        default: break;
+        // TODO: support invariant.start, and experimental.guard here
+        case Intrinsic::assume:
+          // Assumes don't actually alias anything or throw
+          return true;
+        };
+    
     // Handle simple cases by querying alias analysis.
     FunctionModRefBehavior Behavior = AA->getModRefBehavior(CI);
     if (Behavior == FMRB_DoesNotAccessMemory)