[llvm] r211673 - [LICM] Don't create more than one copy of an instruction per loop exit block when sinking.

[Evgeniy Stepanov]

Author: eugenis
Date: Wed Jun 25 02:54:58 2014
New Revision: 211673

URL: http://llvm.org/viewvc/llvm-project?rev=211673&view=rev
Log:
[LICM] Don't create more than one copy of an instruction per loop exit block when sinking.

Fixes exponential compilation complexity in PR19835, caused by
LICM::sink not handling the following pattern well:

f = op g
e = op f, g
d = op e
c = op d, e
b = op c
a = op b, c

When an instruction with N uses is sunk, each of its operands gets N
new uses (all of them - phi nodes). In the example above, if a had 1
use, c would have 2, e would have 4, and g would have 8.

Added:
    llvm/trunk/test/Transforms/LICM/extra-copies.ll
Modified:
    llvm/trunk/lib/Transforms/Scalar/LICM.cpp

Modified: llvm/trunk/lib/Transforms/Scalar/LICM.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LICM.cpp?rev=211673&r1=211672&r2=211673&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/LICM.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/LICM.cpp Wed Jun 25 02:54:58 2014
@@ -550,6 +550,9 @@ void LICM::sink(Instruction &I) {
   SmallPtrSet<BasicBlock *, 32> ExitBlockSet(ExitBlocks.begin(), ExitBlocks.end());
 #endif
 
+  // Clones of this instruction. Don't create more than one per exit block!
+  SmallDenseMap<BasicBlock *, Instruction *, 32> SunkCopies;
+
   // If this instruction is only used outside of the loop, then all users are
   // PHI nodes in exit blocks due to LCSSA form. Just RAUW them with clones of
   // the instruction.
@@ -561,30 +564,37 @@ void LICM::sink(Instruction &I) {
     assert(ExitBlockSet.count(ExitBlock) &&
            "The LCSSA PHI is not in an exit block!");
 
-    Instruction *New = I.clone();
-    ExitBlock->getInstList().insert(ExitBlock->getFirstInsertionPt(), New);
-    if (!I.getName().empty())
-      New->setName(I.getName() + ".le");
-
-    // Build LCSSA PHI nodes for any in-loop operands. Note that this is
-    // particularly cheap because we can rip off the PHI node that we're
-    // replacing for the number and blocks of the predecessors.
-    // OPT: If this shows up in a profile, we can instead finish sinking all
-    // invariant instructions, and then walk their operands to re-establish
-    // LCSSA. That will eliminate creating PHI nodes just to nuke them when
-    // sinking bottom-up.
-    for (User::op_iterator OI = New->op_begin(), OE = New->op_end(); OI != OE;
-         ++OI)
-      if (Instruction *OInst = dyn_cast<Instruction>(*OI))
-        if (Loop *OLoop = LI->getLoopFor(OInst->getParent()))
-          if (!OLoop->contains(PN)) {
-            PHINode *OpPN = PHINode::Create(
-                OInst->getType(), PN->getNumIncomingValues(),
-                OInst->getName() + ".lcssa", ExitBlock->begin());
-            for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
-              OpPN->addIncoming(OInst, PN->getIncomingBlock(i));
-            *OI = OpPN;
-          }
+    Instruction *New;
+    auto It = SunkCopies.find(ExitBlock);
+    if (It != SunkCopies.end()) {
+      New = It->second;
+    } else {
+      New = I.clone();
+      SunkCopies[ExitBlock] = New;
+      ExitBlock->getInstList().insert(ExitBlock->getFirstInsertionPt(), New);
+      if (!I.getName().empty())
+        New->setName(I.getName() + ".le");
+
+      // Build LCSSA PHI nodes for any in-loop operands. Note that this is
+      // particularly cheap because we can rip off the PHI node that we're
+      // replacing for the number and blocks of the predecessors.
+      // OPT: If this shows up in a profile, we can instead finish sinking all
+      // invariant instructions, and then walk their operands to re-establish
+      // LCSSA. That will eliminate creating PHI nodes just to nuke them when
+      // sinking bottom-up.
+      for (User::op_iterator OI = New->op_begin(), OE = New->op_end(); OI != OE;
+           ++OI)
+        if (Instruction *OInst = dyn_cast<Instruction>(*OI))
+          if (Loop *OLoop = LI->getLoopFor(OInst->getParent()))
+            if (!OLoop->contains(PN)) {
+              PHINode *OpPN = PHINode::Create(
+                  OInst->getType(), PN->getNumIncomingValues(),
+                  OInst->getName() + ".lcssa", ExitBlock->begin());
+              for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
+                OpPN->addIncoming(OInst, PN->getIncomingBlock(i));
+              *OI = OpPN;
+            }
+    }
 
     PN->replaceAllUsesWith(New);
     PN->eraseFromParent();

Added: llvm/trunk/test/Transforms/LICM/extra-copies.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LICM/extra-copies.ll?rev=211673&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/LICM/extra-copies.ll (added)
+++ llvm/trunk/test/Transforms/LICM/extra-copies.ll Wed Jun 25 02:54:58 2014
@@ -0,0 +1,29 @@
+; RUN: opt < %s -licm -S | FileCheck %s
+; PR19835
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define i32 @f(i32 %x) {
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %entry, %for.body
+  %storemerge4 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
+  %mul = mul nsw i32 %x, %x
+  %add2 = add nsw i32 %mul, %x
+  %mul3 = add nsw i32 %add2, %mul
+  %inc = add nsw i32 %storemerge4, 1
+  %cmp = icmp slt i32 %inc, 100
+  br i1 %cmp, label %for.body, label %for.end
+
+for.end:                                          ; preds = %for.body
+  %a9.0.lcssa = phi i32 [ %mul3, %for.body ]
+  ret i32 %a9.0.lcssa
+}
+
+; Test that there is exactly one copy of mul nsw i32 %x, %x in the exit block.
+; CHECK: define i32 @f(i32 [[X:%.*]])
+; CHECK: for.end:
+; CHECK-NOT: mul nsw i32 [[X]], [[X]]
+; CHECK: mul nsw i32 [[X]], [[X]]
+; CHECK-NOT: mul nsw i32 [[X]], [[X]]