"Allow LICM to sink or lift loads from constant memory. Also add a test

case for this.

This allows instructions like loads from global variables declared to
be constant to be moved out of loops."

Patch by Stefanus Du Toit!



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@53945 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 28 insertions, 0 deletions

diff --git a/lib/Transforms/Scalar/LICM.cpp b/lib/Transforms/Scalar/LICM.cpp
index f7afaf4..13440c1 100644
--- a/lib/Transforms/Scalar/LICM.cpp
+++ b/lib/Transforms/Scalar/LICM.cpp
@@ -370,6 +370,11 @@ bool LICM::canSinkOrHoistInst(Instruction &I) {
     if (LI->isVolatile())
       return false;        // Don't hoist volatile loads!
 
+    // Loads from constant memory are always safe to move, even if they end up
+    // in the same alias set as something that ends up being modified.
+    if (AA->pointsToConstantMemory(LI->getOperand(0)))
+      return true;
+    
     // Don't hoist loads which have may-aliased stores in loop.
     unsigned Size = 0;
     if (LI->getType()->isSized())
diff --git a/test/Transforms/LICM/2008-07-22-LoadGlobalConstant.ll b/test/Transforms/LICM/2008-07-22-LoadGlobalConstant.ll
new file mode 100644
index 0000000..c3af29b
--- /dev/null
+++ b/test/Transforms/LICM/2008-07-22-LoadGlobalConstant.ll
@@ -0,0 +1,23 @@
+; RUN: llvm-as < %s | opt -licm | llvm-dis | grep -A 1 entry | grep load.*@a
+@a = external constant float*
+
+define void @test(i32 %count) {
+entry:
+        br label %forcond
+
+forcond:
+        %i.0 = phi i32 [ 0, %entry ], [ %inc, %forbody ]
+        %cmp = icmp ult i32 %i.0, %count
+        br i1 %cmp, label %forbody, label %afterfor
+
+forbody:
+        %tmp3 = load float** @a
+        %arrayidx = getelementptr float* %tmp3, i32 %i.0
+        %tmp7 = uitofp i32 %i.0 to float
+        store float %tmp7, float* %arrayidx
+        %inc = add i32 %i.0, 1
+        br label %forcond
+
+afterfor:
+        ret void
+}