Fix a devilish miscompile exposed by block placement. The

updateTerminator code didn't correctly handle EH terminators in one very
specific case. AnalyzeBranch would find no terminator instruction, and
so the fallback in updateTerminator is to assume fallthrough. This is
correct, but the destination of the fallthrough was assumed to be the
first successor.

This is *almost always* true, but in certain cases the loop
transformations will cause the landing pad to be the first successor!
Instead of this brittle logic, actually look through the successors for
a non-landing-pad accessor, and to assert if more than one is found.

This will hopefully fix some (if not all) of the self host miscompiles
with block placement. Thanks to Benjamin Kramer for reporting, Nick
Lewycky for an initial stab at a reduction, and Duncan for endless
advice on EH (which I know nothing about) as well as reviewing the
actual fix.

llvm-svn: 145062

lib/CodeGen/MachineBasicBlock.cpp

@@ -297,8 +297,14 @@ void MachineBasicBlock::updateTerminator() {
         TII->RemoveBranch(*this);
     } else {
       // The block has an unconditional fallthrough. If its successor is not
-      // its layout successor, insert a branch.
-      TBB = *succ_begin();
+      // its layout successor, insert a branch. First we have to locate the
+      // only non-landing-pad successor, as that is the fallthrough block.
+      for (succ_iterator SI = succ_begin(), SE = succ_end(); SI != SE; ++SI) {
+        if ((*SI)->isLandingPad())
+          continue;
+        assert(!TBB && "Found more than one non-landing-pad successor!");
+        TBB = *SI;
+      }
       if (!isLayoutSuccessor(TBB))
         TII->InsertBranch(*this, TBB, 0, Cond, dl);
     }

test/CodeGen/X86/block-placement.ll

@@ -475,3 +475,31 @@ exit:
 }
 
 !2 = metadata !{metadata !"branch_weights", i32 3, i32 1}
+
+declare i32 @__gxx_personality_v0(...)
+
+define void @test_eh_lpad_successor() {
+; Some times the landing pad ends up as the first successor of an invoke block.
+; When this happens, a strange result used to fall out of updateTerminators: we
+; didn't correctly locate the fallthrough successor, assuming blindly that the
+; first one was the fallthrough successor. As a result, we would add an
+; erroneous jump to the landing pad thinking *that* was the default successor.
+; CHECK: test_eh_lpad_successor
+; CHECK: %entry
+; CHECK-NOT: jmp
+; CHECK: %loop
+
+entry:
+  invoke i32 @f() to label %preheader unwind label %lpad
+
+preheader:
+  br label %loop
+
+lpad:
+  %lpad.val = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*)
+          cleanup
+  resume { i8*, i32 } %lpad.val
+
+loop:
+  br label %loop
+}