[PATCH] Take alignment into account in isSafeToLoadUnconditionally

[Artur Pilipenko]

Hi sanjoy,

Take alignment into account in isSafeToLoadUnconditionally. Separated from the D9791.

http://reviews.llvm.org/D10475

Files:
  lib/Analysis/Loads.cpp
  test/Transforms/InstCombine/select.ll

Index: lib/Analysis/Loads.cpp
===================================================================
--- lib/Analysis/Loads.cpp
+++ lib/Analysis/Loads.cpp
@@ -65,6 +65,12 @@
 bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
                                        unsigned Align) {
   const DataLayout &DL = ScanFrom->getModule()->getDataLayout();
+
+  // Require ABI alignment for loads without alignment specification
+  if (Align == 0)
+    Align = DL.getABITypeAlignment(V->getType()->getPointerElementType());
+  assert(isPowerOf2_32(Align));
+
   int64_t ByteOffset = 0;
   Value *Base = V;
   Base = GetPointerBaseWithConstantOffset(V, ByteOffset, DL);
@@ -102,7 +108,7 @@
     if (Align <= BaseAlign) {
       // Check if the load is within the bounds of the underlying object.
       if (ByteOffset + LoadSize <= DL.getTypeAllocSize(BaseType) &&
-          (Align == 0 || (ByteOffset % Align) == 0))
+          ((ByteOffset % Align) == 0))
         return true;
     }
   }
@@ -128,20 +134,28 @@
       return false;
 
     Value *AccessedPtr;
-    if (LoadInst *LI = dyn_cast<LoadInst>(BBI))
+    unsigned AccessedAlign;
+    if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
       AccessedPtr = LI->getPointerOperand();
-    else if (StoreInst *SI = dyn_cast<StoreInst>(BBI))
+      AccessedAlign = LI->getAlignment();
+    } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
       AccessedPtr = SI->getPointerOperand();
-    else
+      AccessedAlign = SI->getAlignment();
+    } else
+      continue;
+
+    Type *AccessedTy = AccessedPtr->getType()->getPointerElementType();
+    if (AccessedAlign == 0)
+      AccessedAlign = DL.getABITypeAlignment(AccessedTy);
+    if (AccessedAlign < Align)
       continue;
 
     // Handle trivial cases.
     if (AccessedPtr == V)
       return true;
 
-    auto *AccessedTy = cast<PointerType>(AccessedPtr->getType());
     if (AreEquivalentAddressValues(AccessedPtr->stripPointerCasts(), V) &&
-        LoadSize <= DL.getTypeStoreSize(AccessedTy->getElementType()))
+        LoadSize <= DL.getTypeStoreSize(AccessedTy))
       return true;
   }
   return false;
Index: test/Transforms/InstCombine/select.ll
===================================================================
--- test/Transforms/InstCombine/select.ll
+++ test/Transforms/InstCombine/select.ll
@@ -1296,6 +1296,23 @@
   ret i32 %v
 }
 
+define i32 @test78_neg(i1 %flag, i32* %x, i32* %y, i32* %z) {
+; The same as @test78 but we can't speculate the load because it can trap
+; if under-aligned.
+; CHECK-LABEL: @test78_neg(
+; CHECK: %p = select i1 %flag, i32* %x, i32* %y
+; CHECK-NEXT: %v = load i32, i32* %p, align 16
+; CHECK-NEXT: ret i32 %v
+entry:
+  store i32 0, i32* %x
+  store i32 0, i32* %y
+  ; Block forwarding by storing to %z which could alias either %x or %y.
+  store i32 42, i32* %z
+  %p = select i1 %flag, i32* %x, i32* %y
+  %v = load i32, i32* %p, align 16
+  ret i32 %v
+}
+
 define float @test79(i1 %flag, float* %x, i32* %y, i32* %z) {
 ; Test that we can speculate the loads around the select even when we can't
 ; fold the load completely away.

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