[LICM] Hoist assumes out of loops

[reames]

Summary

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.

[mkazantsev]

test/Transforms/LICM/assume.ll

36 ↗

(On Diff #159406)

Could you please add another test where assume uses a Phi or another unhoistable value from the loop, and therefore cannot be hoisted?

[reames]

Rebased on top of newly submitted tests to address review comment

[reames]

ping

[mkazantsev]

LGTM

llvm/trunk/lib/Transforms/Scalar/LICM.cpp

if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
     // Don't sink or hoist dbg info; it's legal, but not useful.
     if (isa<DbgInfoIntrinsic>(I))
       return false;
 
     // Don't sink calls which can throw.
     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)
       return true;
     if (AliasAnalysis::onlyReadsMemory(Behavior)) {
       // A readonly argmemonly function only reads from memory pointed to by
       // it's arguments with arbitrary offsets.  If we can prove there are no
       // writes to this memory in the loop, we can hoist or sink.

llvm/trunk/test/Transforms/LICM/assume.ll

 
 for.end104:
   ret void
 }
 
 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
 
 loop:
   %x = phi i32 [ 0, %entry ], [ %x.inc, %loop ]
   call void @llvm.assume(i1 %cond)
   %val = load i32, i32* %ptr