[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

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)

test/Transforms/LICM/assume.ll

@@ -36,9 +36,9 @@ for.end104:
 define void @f_1(i1 %cond, i32* %ptr) {
 ; CHECK-LABEL: @f_1(
 ; CHECK-LABEL: entry:
+; CHECK: call void @llvm.assume(i1 %cond)
 ; CHECK: %val = load i32, i32* %ptr
 ; CHECK-LABEL: loop:
-; CHECK: call void @llvm.assume(i1 %cond)
 
 entry:
   br label %loop