[LCSSA] Skip blocks in sub-loops when scanning for uses.

[fhahn]

Summary

Scanning blocks in sub-loops for uses is unnecessary, as they were
already handled while dealing with the containing sub-loop.

This speeds up LCSSA for highly nested loops. For the test case in PR37202, it
halves the time spent in LCSSA. In cases were we won't be able to skip
any blocks, the additional lookup should be negligible.

Time-passes without this patch for test case from PR37202:

Total Execution Time: 48.5505 seconds (48.5511 wall clock)

 ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
10.0822 ( 21.0%)   0.1406 ( 27.0%)  10.2228 ( 21.1%)  10.2228 ( 21.1%)  Loop-Closed SSA Form Pass
10.0417 ( 20.9%)   0.1467 ( 28.2%)  10.1884 ( 21.0%)  10.1890 ( 21.0%)  Loop-Closed SSA Form Pass #2
 4.2703 (  8.9%)   0.0040 (  0.8%)   4.2742 (  8.8%)   4.2742 (  8.8%)  Unswitch loops
 2.7376 (  5.7%)   0.0229 (  4.4%)   2.7605 (  5.7%)   2.7611 (  5.7%)  Loop-Closed SSA Form Pass #5
 2.7332 (  5.7%)   0.0214 (  4.1%)   2.7546 (  5.7%)   2.7546 (  5.7%)  Loop-Closed SSA Form Pass #3
 2.7088 (  5.6%)   0.0230 (  4.4%)   2.7319 (  5.6%)   2.7324 (  5.6%)  Loop-Closed SSA Form Pass #4
 2.6855 (  5.6%)   0.0236 (  4.5%)   2.7091 (  5.6%)   2.7090 (  5.6%)  Loop-Closed SSA Form Pass #6
 2.1648 (  4.5%)   0.0018 (  0.4%)   2.1666 (  4.5%)   2.1664 (  4.5%)  Unroll loops
 1.8371 (  3.8%)   0.0009 (  0.2%)   1.8379 (  3.8%)   1.8380 (  3.8%)  Value Propagation
 1.8149 (  3.8%)   0.0021 (  0.4%)   1.8170 (  3.7%)   1.8169 (  3.7%)  Loop Invariant Code Motion
 1.6755 (  3.5%)   0.0226 (  4.3%)   1.6981 (  3.5%)   1.6980 (  3.5%)  Loop-Closed SSA Form Pass #7

Time-passes with this patch

Total Execution Time: 29.9285 seconds (29.9276 wall clock)

 ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
 5.2786 ( 17.7%)   0.0021 (  1.2%)   5.2806 ( 17.6%)   5.2808 ( 17.6%)  Unswitch loops
 4.3739 ( 14.7%)   0.0303 ( 18.1%)   4.4042 ( 14.7%)   4.4042 ( 14.7%)  Loop-Closed SSA Form Pass
 4.2658 ( 14.3%)   0.0192 ( 11.5%)   4.2850 ( 14.3%)   4.2851 ( 14.3%)  Loop-Closed SSA Form Pass #2
 2.2307 (  7.5%)   0.0013 (  0.8%)   2.2320 (  7.5%)   2.2318 (  7.5%)  Loop Invariant Code Motion
 2.0888 (  7.0%)   0.0012 (  0.7%)   2.0900 (  7.0%)   2.0897 (  7.0%)  Unroll loops
 1.6761 (  5.6%)   0.0013 (  0.8%)   1.6774 (  5.6%)   1.6774 (  5.6%)  Value Propagation
 1.3686 (  4.6%)   0.0029 (  1.8%)   1.3716 (  4.6%)   1.3714 (  4.6%)  Induction Variable Simplification
 1.1457 (  3.8%)   0.0010 (  0.6%)   1.1468 (  3.8%)   1.1468 (  3.8%)  Loop-Closed SSA Form Pass #4
 1.1384 (  3.8%)   0.0005 (  0.3%)   1.1389 (  3.8%)   1.1389 (  3.8%)  Loop-Closed SSA Form Pass #6
 1.1360 (  3.8%)   0.0027 (  1.6%)   1.1387 (  3.8%)   1.1387 (  3.8%)  Loop-Closed SSA Form Pass #5
 1.1331 (  3.8%)   0.0010 (  0.6%)   1.1341 (  3.8%)   1.1340 (  3.8%)  Loop-Closed SSA Form Pass #3

[efriedma]

llvm/lib/Transforms/Utils/LCSSA.cpp

328

The relevant property here is that the sub-loops are already in LCSSA form, right? The documentation for formLCSSA doesn't actually say that explicitly... granted, it's probably true in the cases we care about.

[fhahn]

state explicitly that sub-loops must be in LCSSA form already for formLCSSA()

[fhahn]

llvm/lib/Transforms/Utils/LCSSA.cpp

328

That's a better way of putting it, yes! The formLCSSA uses in LCSSA.cpp guarantee that already and I checked the other 3 uses in LoopSimplify.cpp and SimpleLoopUnswitch.cpp and they either call formLCSSA depth first for their sub-loops or assert that the subloops are in LCSSA form.

[efriedma]

LGTM

[davide]

LGTM, just a minor comment (food for thought).

llvm/lib/Transforms/Utils/LCSSA.cpp

328

This property of sub loops should always be true, maybe there's a way of verifying this?

[fhahn]

llvm/lib/Transforms/Utils/LCSSA.cpp

328

Yes, I think we can just check isRecursivelyLCSSAForm on all sub-loops. This is quite expensive, which is why I didn't put it in this patch. But I'll create a follow up, where it is under EXPENSIVE_CHECKS

llvm/include/llvm/Transforms/Utils/LoopUtils.h

 /// Returns true if any modifications are made.
 bool formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
                               DominatorTree &DT, LoopInfo &LI);
 
 /// Put loop into LCSSA form.
 ///
 /// Looks at all instructions in the loop which have uses outside of the
 /// current loop. For each, an LCSSA PHI node is inserted and the uses outside
-/// the loop are rewritten to use this node.
+/// the loop are rewritten to use this node. Sub-loops must be in LCSSA form
+/// already.
 ///
 /// LoopInfo and DominatorTree are required and preserved.
 ///
 /// If ScalarEvolution is passed in, it will be preserved.
 ///
 /// Returns true if any modifications are made to the loop.
 bool formLCSSA(Loop &L, DominatorTree &DT, LoopInfo *LI, ScalarEvolution *SE);
 

llvm/lib/Transforms/Utils/LCSSA.cpp

bool llvm::formLCSSA(Loop &L, DominatorTree &DT, LoopInfo *LI,
   // walking the dominator tree upwards until we hit a loop header.
   computeBlocksDominatingExits(L, DT, ExitBlocks, BlocksDominatingExits);
 
   SmallVector<Instruction *, 8> Worklist;
 
   // Look at all the instructions in the loop, checking to see if they have uses
   // outside the loop.  If so, put them into the worklist to rewrite those uses.
   for (BasicBlock *BB : BlocksDominatingExits) {
+    // Skip blocks that are part of any sub-loops, they must be in LCSSA
+    // already.
+    if (LI->getLoopFor(BB) != &L)
+      continue;
     for (Instruction &I : *BB) {
       // Reject two common cases fast: instructions with no uses (like stores)
       // and instructions with one use that is in the same block as this.
       if (I.use_empty() ||
           (I.hasOneUse() && I.user_back()->getParent() == BB &&
            !isa<PHINode>(I.user_back())))
         continue;