Fix miscompile due to StackColoring incorrectly merging stack slots (PR15707)

IR optimisation passes can result in a basic block that contains:

  llvm.lifetime.start(%buf)
  ...
  llvm.lifetime.end(%buf)
  ...
  llvm.lifetime.start(%buf)

Before this change, calculateLiveIntervals() was ignoring the second
lifetime.start() and was regarding %buf as being dead from the
lifetime.end() through to the end of the basic block.  This can cause
StackColoring to incorrectly merge %buf with another stack slot.

Fix by removing the incorrect Starts[pos].isValid() and
Finishes[pos].isValid() checks.

Just doing:
      Starts[pos] = Indexes->getMBBStartIdx(MBB);
      Finishes[pos] = Indexes->getMBBEndIdx(MBB);
unconditionally would be enough to fix the bug, but it causes some
test failures due to stack slots not being merged when they were
before.  So, in order to keep the existing tests passing, treat LiveIn
and LiveOut separately rather than approximating the live ranges by
merging LiveIn and LiveOut.

This fixes PR15707.
Patch by Mark Seaborn.

llvm-svn: 181922

2 files changed, 30 insertions, 11 deletions

diff --git a/llvm/lib/CodeGen/StackColoring.cpp b/llvm/lib/CodeGen/StackColoring.cpp
index 90b93aaa724..5f4c68bc0e1 100644
--- a/llvm/lib/CodeGen/StackColoring.cpp
+++ b/llvm/lib/CodeGen/StackColoring.cpp
@@ -429,17 +429,14 @@ void StackColoring::calculateLiveIntervals(unsigned NumSlots) {
     }
 
     // Create the interval of the blocks that we previously found to be 'alive'.
-    BitVector Alive = BlockLiveness[MBB].LiveIn;
-    Alive |= BlockLiveness[MBB].LiveOut;
-
-    if (Alive.any()) {
-      for (int pos = Alive.find_first(); pos != -1;
-           pos = Alive.find_next(pos)) {
-        if (!Starts[pos].isValid())
-          Starts[pos] = Indexes->getMBBStartIdx(MBB);
-        if (!Finishes[pos].isValid())
-          Finishes[pos] = Indexes->getMBBEndIdx(MBB);
-      }
+    BlockLifetimeInfo &MBBLiveness = BlockLiveness[MBB];
+    for (int pos = MBBLiveness.LiveIn.find_first(); pos != -1;
+         pos = MBBLiveness.LiveIn.find_next(pos)) {
+      Starts[pos] = Indexes->getMBBStartIdx(MBB);
+    }
+    for (int pos = MBBLiveness.LiveOut.find_first(); pos != -1;
+         pos = MBBLiveness.LiveOut.find_next(pos)) {
+      Finishes[pos] = Indexes->getMBBEndIdx(MBB);
     }
 
     for (unsigned i = 0; i < NumSlots; ++i) {
diff --git a/llvm/test/CodeGen/X86/StackColoring.ll b/llvm/test/CodeGen/X86/StackColoring.ll
index f8ae74f292d..6c0f00d17d5 100644
--- a/llvm/test/CodeGen/X86/StackColoring.ll
+++ b/llvm/test/CodeGen/X86/StackColoring.ll
@@ -350,6 +350,28 @@ bb3:
   ret i32 0
 }
 
+
+; Regression test for PR15707.  %buf1 and %buf2 should not be merged
+; in this test case.
+;YESCOLOR: myCall_pr15707
+;YESCOLOR: subq $200008, %rsp
+;NOCOLOR: myCall_pr15707
+;NOCOLOR: subq $200008, %rsp
+define void @myCall_pr15707() {
+  %buf1 = alloca i8, i32 100000, align 16
+  %buf2 = alloca i8, i32 100000, align 16
+
+  call void @llvm.lifetime.start(i64 -1, i8* %buf1)
+  call void @llvm.lifetime.end(i64 -1, i8* %buf1)
+
+  call void @llvm.lifetime.start(i64 -1, i8* %buf1)
+  call void @llvm.lifetime.start(i64 -1, i8* %buf2)
+  %result1 = call i32 @foo(i32 0, i8* %buf1)
+  %result2 = call i32 @foo(i32 0, i8* %buf2)
+  ret void
+}
+
+
 ; Check that we don't assert and crash even when there are allocas
 ; outside the declared lifetime regions.
 ;YESCOLOR: bad_range