Don't recompute LCSSA after loop-unrolling when possible.

Summary:
Currently we always recompute LCSSA for outer loops after unrolling an
inner loop. That leads to compile time problem when we have big loop
nests, and we can solve it by avoiding unnecessary work. For instance,
if w eonly do partial unrolling, we don't break LCSSA, so we don't need
to rebuild it. Also, if all exits from the inner loop are inside the
enclosing loop, then complete unrolling won't break LCSSA either.

I replaced unconditional LCSSA recomputation with conditional recomputation +
unconditional assert and added several tests, which were failing when I
experimented with it.

Soon I plan to follow up with a similar patch for recalculation of dominators
tree.

Reviewers: hfinkel, dexonsmith, bogner, joker.eph, chandlerc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14526

llvm-svn: 253126

lib/Transforms/Utils/LoopUnroll.cpp

@@ -221,6 +221,12 @@ bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
 
   // Are we eliminating the loop control altogether?
   bool CompletelyUnroll = Count == TripCount;
+  SmallVector<BasicBlock *, 4> ExitBlocks;
+  L->getExitBlocks(ExitBlocks);
+  Loop *ParentL = L->getParentLoop();
+  bool AllExitsAreInsideParentLoop = !ParentL ||
+      std::all_of(ExitBlocks.begin(), ExitBlocks.end(),
+                  [&](BasicBlock *BB) { return ParentL->contains(BB); });
 
   // We assume a run-time trip count if the compiler cannot
   // figure out the loop trip count and the unroll-runtime
@@ -554,7 +560,11 @@ bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
         while (OuterL->getParentLoop() != LatchLoop)
           OuterL = OuterL->getParentLoop();
 
-      formLCSSARecursively(*OuterL, *DT, LI, SE);
+      if (CompletelyUnroll && !AllExitsAreInsideParentLoop)
+        formLCSSARecursively(*OuterL, *DT, LI, SE);
+      else
+        assert(OuterL->isLCSSAForm(*DT) &&
+               "Loops should be in LCSSA form after loop-unroll.");
     }
   }
 

test/Transforms/LoopUnroll/rebuild_lcssa.ll

@@ -0,0 +1,119 @@
+; RUN: opt < %s -loop-unroll -S | FileCheck %s
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+
+; This test shows how unrolling an inner loop could break LCSSA for an outer
+; loop, and there is no cheap way to recover it.
+;
+; In this case the inner loop, L3, is being unrolled. It only runs one
+; iteration, so unrolling basically means replacing
+;   br i1 true, label %exit, label %L3_header
+; with
+;   br label %exit
+;
+; However, this change messes up the loops structure: for instance, block
+; L3_body no longer belongs to L2. It becomes an exit block for L2, so LCSSA
+; phis for definitions in L2 should now be placed there. In particular, we need
+; to insert such a definition for %y1.
+
+; CHECK-LABEL: @foo1
+define void @foo1() {
+entry:
+  br label %L1_header
+
+L1_header:
+  br label %L2_header
+
+L2_header:
+  %y1 = phi i64 [ undef, %L1_header ], [ %x.lcssa, %L2_latch ]
+  br label %L3_header
+
+L3_header:
+  %y2 = phi i64 [ 0, %L3_latch ], [ %y1, %L2_header ]
+  %x = add i64 undef, -1
+  br i1 true, label %L2_latch, label %L3_body
+
+L2_latch:
+  %x.lcssa = phi i64 [ %x, %L3_header ]
+  br label %L2_header
+
+; CHECK:      L3_body:
+; CHECK-NEXT:   %y1.lcssa = phi i64 [ %y1, %L3_header ]
+L3_body:
+  store i64 %y1, i64* undef
+  br i1 false, label %L3_latch, label %L1_latch
+
+L3_latch:
+  br i1 true, label %exit, label %L3_header
+
+L1_latch:
+  %y.lcssa = phi i64 [ %y2, %L3_body ]
+  br label %L1_header
+
+exit:
+  ret void
+}
+
+; Additional tests for some corner cases.
+;
+; CHECK-LABEL: @foo2
+define void @foo2() {
+entry:
+  br label %L1_header
+
+L1_header:
+  br label %L2_header
+
+L2_header:
+  %a = phi i64 [ undef, %L1_header ], [ %dec_us, %L3_header ]
+  br label %L3_header
+
+L3_header:
+  %b = phi i64 [ 0, %L3_latch ], [ %a, %L2_header ]
+  %dec_us = add i64 undef, -1
+  br i1 true, label %L2_header, label %L3_break_to_L1
+
+; CHECK:      L3_break_to_L1:
+; CHECK-NEXT:   %a.lcssa = phi i64 [ %a, %L3_header ]
+L3_break_to_L1:
+  br i1 false, label %L3_latch, label %L1_latch
+
+L1_latch:
+  %b_lcssa = phi i64 [ %b, %L3_break_to_L1 ]
+  br label %L1_header
+
+L3_latch:
+  br i1 true, label %Exit, label %L3_header
+
+Exit:
+  ret void
+}
+
+; CHECK-LABEL: @foo3
+define void @foo3() {
+entry:
+  br label %L1_header
+
+L1_header:
+  %a = phi i8* [ %b, %L1_latch ], [ null, %entry ]
+  br i1 undef, label %L2_header, label %L1_latch
+
+L2_header:
+  br i1 undef, label %L2_latch, label %L1_latch
+
+; CHECK:      L2_latch:
+; CHECK-NEXT:   %a.lcssa = phi i8* [ %a, %L2_header ]
+L2_latch:
+  br i1 true, label %L2_exit, label %L2_header
+
+L1_latch:
+  %b = phi i8* [ undef, %L1_header ], [ null, %L2_header ]
+  br label %L1_header
+
+L2_exit:
+  %a_lcssa1 = phi i8* [ %a, %L2_latch ]
+  br label %Exit
+
+Exit:
+  %a_lcssa2 = phi i8* [ %a_lcssa1, %L2_exit ]
+  ret void
+}