[InstCombine] Remove trivially empty lifetime start/end ranges.

Summary:
Some passes may open up opportunities for optimizations, leaving empty
lifetime start/end ranges. For example, with the following code:

    void foo(char *, char *);
    void bar(int Size, bool flag) {
      for (int i = 0; i < Size; ++i) {
        char text[1];
        char buff[1];
        if (flag)
          foo(text, buff); // BBFoo
      }
    }

the loop unswitch pass will create 2 versions of the loop, one with
flag==true, and the other one with flag==false, but always leaving
the BBFoo basic block, with lifetime ranges covering the scope of the for
loop. Simplify CFG will then remove BBFoo in the case where flag==false,
but will leave the lifetime markers.

This patch teaches InstCombine to remove trivially empty lifetime marker
ranges, that is ranges ending right after they were started (ignoring
debug info or other lifetime markers in the range).

This fixes PR24598: excessive compile time after r234581.

Reviewers: reames, chandlerc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13305

llvm-svn: 249018

1 files changed, 23 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 8f3deacf248..b1a20ff3e6e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -1433,6 +1433,29 @@ Instruction *InstCombiner::visitCallInst(CallInst &CI) {
       return EraseInstFromFunction(CI);
     break;
   }
+  case Intrinsic::lifetime_start: {
+    // Remove trivially empty lifetime_start/end ranges, i.e. a start
+    // immediately followed by an end (ignoring debuginfo or other
+    // lifetime markers in between).
+    BasicBlock::iterator BI = II, BE = II->getParent()->end();
+    for (++BI; BI != BE; ++BI) {
+      if (IntrinsicInst *LTE = dyn_cast<IntrinsicInst>(BI)) {
+        if (isa<DbgInfoIntrinsic>(LTE) ||
+            LTE->getIntrinsicID() == Intrinsic::lifetime_start)
+          continue;
+        if (LTE->getIntrinsicID() == Intrinsic::lifetime_end) {
+          if (II->getOperand(0) == LTE->getOperand(0) &&
+              II->getOperand(1) == LTE->getOperand(1)) {
+            EraseInstFromFunction(*LTE);
+            return EraseInstFromFunction(*II);
+          }
+          continue;
+        }
+      }
+      break;
+    }
+    break;
+  }
   case Intrinsic::assume: {
     // Canonicalize assume(a && b) -> assume(a); assume(b);
     // Note: New assumption intrinsics created here are registered by