EarlyCSE should ignore calls to @llvm.assume

EarlyCSE uses a simple generation scheme for handling memory-based
dependencies, and calls to @llvm.assume (which are marked as writing to memory
to ensure the preservation of control dependencies) disturb that scheme
unnecessarily. Skipping calls to @llvm.assume is legal, and the alternative
(adding AA calls in EarlyCSE) is likely undesirable (we have GVN for that).

Fixes PR21448.

llvm-svn: 221175

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
index 131a8cb3dcb..6f166b8ee7f 100644
--- a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
+++ b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
@@ -21,6 +21,8 @@
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Instructions.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/PatternMatch.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/RecyclingAllocator.h"
@@ -28,6 +30,7 @@
 #include "llvm/Transforms/Utils/Local.h"
 #include <deque>
 using namespace llvm;
+using namespace llvm::PatternMatch;
 
 #define DEBUG_TYPE "early-cse"
 
@@ -435,6 +438,15 @@ bool EarlyCSE::processNode(DomTreeNode *Node) {
       continue;
     }
 
+    // Skip assume intrinsics, they don't really have side effects (although
+    // they're marked as such to ensure preservation of control dependencies),
+    // and this pass will not disturb any of the assumption's control
+    // dependencies.
+    if (match(Inst, m_Intrinsic<Intrinsic::assume>())) {
+      DEBUG(dbgs() << "EarlyCSE skipping assumption: " << *Inst << '\n');
+      continue;
+    }
+
     // If the instruction can be simplified (e.g. X+0 = X) then replace it with
     // its simpler value.
     if (Value *V = SimplifyInstruction(Inst, DL, TLI, DT, AT)) {