[Tests] Give meaningful names to blocks in LoopUnroll/pr18861.ll, add a description of what's going on.

llvm-svn: 249173

llvm/test/Transforms/LoopUnroll/pr18861.ll

@@ -2,30 +2,54 @@
 
 @b = external global i32, align 4
 
+; Test that LoopUnroll does not break LCSSA form.
+;
+; In this function we have a following CFG:
+;            ( entry )
+;                |
+;                v
+;         ( outer.header ) <--
+;                |             \
+;                v              |
+;     --> ( inner.header )      |
+;   /       /          \        |
+;   \      /            \       |
+;    \    v              v     /
+;  ( inner.latch )   ( outer.latch )
+;         |
+;         v
+;     ( exit )
+;
+; When the inner loop is unrolled, we inner.latch block has only one
+; predecessor and one successor, so it can be merged with exit block.
+; During the merge, however, we remove an LCSSA definition for
+; %storemerge1.lcssa, breaking LCSSA form for the outer loop.
+
 ; Function Attrs: nounwind uwtable
 define void @fn1() #0 {
 entry:
-  br label %for.cond1thread-pre-split
+  br label %outer.header
 
-for.cond1thread-pre-split:                        ; preds = %for.inc5, %entry
-  %storemerge1 = phi i32 [ 0, %entry ], [ %inc9, %for.inc5 ]
-  br label %for.cond2
+outer.header:                                     ; preds = %outer.latch, %entry
+  %storemerge1 = phi i32 [ 0, %entry ], [ %inc9, %outer.latch ]
+  br label %inner.header
 
-for.cond2:                                        ; preds = %for.body3, %for.cond1thread-pre-split
-  %storemerge = phi i32 [ %add, %for.body3 ], [ 0, %for.cond1thread-pre-split ]
+inner.header:                                     ; preds = %inner.latch, %outer.header
+  %storemerge = phi i32 [ %add, %inner.latch ], [ 0, %outer.header ]
   %cmp = icmp slt i32 %storemerge, 1
-  br i1 %cmp, label %for.body3, label %for.inc5
+  br i1 %cmp, label %inner.latch, label %outer.latch
 
-for.body3:                                        ; preds = %for.cond2
+inner.latch:                                      ; preds = %inner.header
   %tobool4 = icmp eq i32 %storemerge, 0
   %add = add nsw i32 %storemerge, 1
-  br i1 %tobool4, label %for.cond2, label %if.then
+  br i1 %tobool4, label %inner.header, label %exit
 
-if.then:                                          ; preds = %for.body3
-  store i32 %storemerge1, i32* @b, align 4
+exit:                                             ; preds = %inner.latch
+  %storemerge1.lcssa = phi i32 [ %storemerge1, %inner.latch ]
+  store i32 %storemerge1.lcssa, i32* @b, align 4
   ret void
 
-for.inc5:                                         ; preds = %for.cond2
+outer.latch:                                      ; preds = %inner.header
   %inc9 = add nsw i32 %storemerge1, 1
-  br label %for.cond1thread-pre-split
+  br label %outer.header
 }