[llvm] r234651 - [RewriteStatepointsForGC] Preprocess the IR to remove unreachable blocks and single entry phis

[Philip Reames]

Author: reames
Date: Fri Apr 10 17:07:04 2015
New Revision: 234651

URL: http://llvm.org/viewvc/llvm-project?rev=234651&view=rev
Log:
[RewriteStatepointsForGC] Preprocess the IR to remove unreachable blocks and single entry phis

Two related small changes:

    Various dominance based queries about liveness can get confused if we're talking about unreachable blocks. To avoid reasoning about such cases, just remove them before rewriting statepoints.
    Remove single entry phis (likely left behind by LCSSA) to reduce the number of live values.

Both of these are motivated by http://reviews.llvm.org/D8674 which will be submitted shortly.

Differential Revision: http://reviews.llvm.org/D8675


Added:
    llvm/trunk/test/Transforms/RewriteStatepointsForGC/preprocess.ll
Modified:
    llvm/trunk/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

Modified: llvm/trunk/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp?rev=234651&r1=234650&r2=234651&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp Fri Apr 10 17:07:04 2015
@@ -2054,18 +2054,46 @@ bool RewriteStatepointsForGC::runOnFunct
   if (!shouldRewriteStatepointsIn(F))
     return false;
 
-  // Gather all the statepoints which need rewritten.
+  DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+  
+  // Gather all the statepoints which need rewritten.  Be careful to only
+  // consider those in reachable code since we need to ask dominance queries
+  // when rewriting.  We'll delete the unreachable ones in a moment.
   SmallVector<CallSite, 64> ParsePointNeeded;
+  SmallVector<CallSite, 16> UnreachableStatepoints;
   for (Instruction &I : inst_range(F)) {
     // TODO: only the ones with the flag set!
-    if (isStatepoint(I))
-      ParsePointNeeded.push_back(CallSite(&I));
+    if (isStatepoint(I)) {
+      if (DT.isReachableFromEntry(I.getParent()))
+        ParsePointNeeded.push_back(CallSite(&I));
+      else
+        UnreachableStatepoints.push_back(CallSite(&I));
+    }
   }
 
+  bool MadeChange = false;
+  
+  // Delete any unreachable statepoints so that we don't have unrewritten
+  // statepoints surviving this pass.  This makes testing easier and the
+  // resulting IR less confusing to human readers.  Rather than be fancy, we
+  // just reuse a utility function which removes the unreachable blocks.
+  if (!UnreachableStatepoints.empty())
+    MadeChange |= removeUnreachableBlocks(F);
+
   // Return early if no work to do.
   if (ParsePointNeeded.empty())
-    return false;
+    return MadeChange;
 
-  DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
-  return insertParsePoints(F, DT, this, ParsePointNeeded);
+  // As a prepass, go ahead and aggressively destroy single entry phi nodes.
+  // These are created by LCSSA.  They have the effect of increasing the size
+  // of liveness sets for no good reason.  It may be harder to do this post
+  // insertion since relocations and base phis can confuse things.
+  for (BasicBlock &BB : F)
+    if (BB.getUniquePredecessor()) {
+      MadeChange = true;
+      FoldSingleEntryPHINodes(&BB);
+    }
+
+  MadeChange |= insertParsePoints(F, DT, this, ParsePointNeeded);
+  return MadeChange;
 }

Added: llvm/trunk/test/Transforms/RewriteStatepointsForGC/preprocess.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/RewriteStatepointsForGC/preprocess.ll?rev=234651&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/RewriteStatepointsForGC/preprocess.ll (added)
+++ llvm/trunk/test/Transforms/RewriteStatepointsForGC/preprocess.ll Fri Apr 10 17:07:04 2015
@@ -0,0 +1,65 @@
+; RUN: opt -rewrite-statepoints-for-gc -S < %s | FileCheck %s
+
+declare void @consume(...)
+
+; Test to make sure we destroy LCSSA's single entry phi nodes before
+; running liveness
+define void @test6(i64 addrspace(1)* %obj) gc "statepoint-example" {
+; CHECK-LABEL: @test6
+entry:
+  br label %next
+
+next:
+; CHECK-LABEL: next:
+; CHECK-NEXT: gc.statepoint
+; CHECK-NEXT: gc.relocate
+; CHECK-NEXT: @consume(i64 addrspace(1)* %obj.relocated)
+; CHECK-NEXT: @consume(i64 addrspace(1)* %obj.relocated)
+  %obj2 = phi i64 addrspace(1)* [ %obj, %entry ]
+  call i32 (void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(void ()* @foo, i32 0, i32 0, i32 0)
+  call void (...)* @consume(i64 addrspace(1)* %obj2)
+  call void (...)* @consume(i64 addrspace(1)* %obj)
+  ret void
+}
+
+declare void @some_call(i64 addrspace(1)*)
+
+; Need to delete unreachable gc.statepoint call
+define void @test7() gc "statepoint-example" {
+; CHECK-LABEL: test7
+; CHECK-NOT: gc.statepoint
+  ret void
+
+unreached:
+  %obj = phi i64 addrspace(1)* [null, %unreached]
+  call i32 (void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(void ()* @foo, i32 0, i32 0, i32 0)
+  call void (...)* @consume(i64 addrspace(1)* %obj)
+  br label %unreached
+}
+
+; Need to delete unreachable gc.statepoint invoke - tested seperately given
+; a correct implementation could only remove the instructions, not the block
+define void @test8() gc "statepoint-example" {
+; CHECK-LABEL: test8
+; CHECK-NOT: gc.statepoint
+  ret void
+
+unreached:
+  invoke i32 (void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(void ()* @foo, i32 0, i32 0, i32 0)
+          to label %normal_return unwind label %exceptional_return
+
+normal_return:                                    ; preds = %entry
+  ret void
+
+exceptional_return:                               ; preds = %entry
+  %landing_pad4 = landingpad { i8*, i32 } personality i32 ()* undef
+          cleanup
+  ret void
+}
+
+declare void @foo()
+; Bound the last check-not
+; CHECK-LABEL: @foo
+
+declare i32 @llvm.experimental.gc.statepoint.p0f_isVoidf(void ()*, i32, i32, ...)
+