Throttle back tail duplication to avoid creating really ugly sequences of code.

For Transforms/TailDup/if-tail-dup.ll, f.e., it produces:

_foo:
        movl 8(%esp), %eax
        movl 4(%esp), %ecx
        testl $1, %ecx
        je LBB1_2       #cond_next
LBB1_1: #cond_true
        movl $1, (%eax)
LBB1_2: #cond_next
        testl $2, %ecx
        je LBB1_4       #cond_next10
LBB1_3: #cond_true6
        movl $1, 4(%eax)
LBB1_4: #cond_next10
        testl $4, %ecx
        je LBB1_6       #cond_next18
LBB1_5: #cond_true14
        movl $1, 8(%eax)
LBB1_6: #cond_next18
        testl $8, %ecx
        je LBB1_8       #return
LBB1_7: #cond_true22
        movl $1, 12(%eax)
        ret
LBB1_8: #return
        ret

instead of:

_foo:
        movl 4(%esp), %eax
        testl $2, %eax
        sete %cl
        movl 8(%esp), %edx
        testl $1, %eax
        je LBB1_2       #cond_next
LBB1_1: #cond_true
        movl $1, (%edx)
        testb %cl, %cl
        jne LBB1_4      #cond_next10
        jmp LBB1_3      #cond_true6
LBB1_2: #cond_next
        testb %cl, %cl
        jne LBB1_4      #cond_next10
LBB1_3: #cond_true6
        movl $1, 4(%edx)
        testl $4, %eax
        je LBB1_6       #cond_next18
        jmp LBB1_5      #cond_true14
LBB1_4: #cond_next10
        testl $4, %eax
        je LBB1_6       #cond_next18
LBB1_5: #cond_true14
        movl $1, 8(%edx)
        testl $8, %eax
        je LBB1_8       #return
        jmp LBB1_7      #cond_true22
LBB1_6: #cond_next18
        testl $8, %eax
        je LBB1_8       #return
LBB1_7: #cond_true22
        movl $1, 12(%edx)
        ret
LBB1_8: #return
        ret

llvm-svn: 30158

1 files changed, 19 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/TailDuplication.cpp b/llvm/lib/Transforms/Scalar/TailDuplication.cpp
index acf0db0ebe4..43aaced3789 100644
--- a/llvm/lib/Transforms/Scalar/TailDuplication.cpp
+++ b/llvm/lib/Transforms/Scalar/TailDuplication.cpp
@@ -127,6 +127,25 @@ bool TailDup::shouldEliminateUnconditionalBranch(TerminatorInst *TI) {
     for (; PI != PE; ++PI)
       if (TooMany-- == 0) return false;
   }
+  
+  // Finally, if this unconditional branch is a fall-through, be careful about
+  // tail duplicating it.  In particular, we don't want to taildup it if the
+  // original block will still be there after taildup is completed: doing so
+  // would eliminate the fall-through, requiring unconditional branches.
+  Function::iterator DestI = Dest;
+  if (&*--DestI == BI->getParent()) {
+    // The uncond branch is a fall-through.  Tail duplication of the block is
+    // will eliminate the fall-through-ness and end up cloning the terminator
+    // at the end of the Dest block.  Since the original Dest block will
+    // continue to exist, this means that one or the other will not be able to
+    // fall through.  One typical example that this helps with is code like:
+    // if (a)
+    //   foo();
+    // if (b)
+    //   foo();
+    // Cloning the 'if b' block into the end of the first foo block is messy.
+    return false;
+  }
 
   return true;
 }