[CodeGen][IfCvt] Don't re-ifcvt blocks with unanalyzable terminators.
If we couldn't analyze its terminator (i.e., it's an indirectbr, or some
other weirdness), we can't safely re-if-convert a predicated block,
because we can't tell whether the predicated terminator can
fallthrough (it does).
Currently, we would completely ignore the fallthrough successor. In
the added testcase, this means we used to generate:
...
@ %entry:
cmp r5, #21
ittt ne
@ %cc1f:
cmpne r7, #42
@ %cc2t:
strne.w r5, [r8]
movne pc, r10
@ %cc1t:
...
Whereas the successor of %cc1f was originally %bb1.
With the fix, we get the correct:
...
@ %entry:
cmp r5, #21
itt eq
@ %cc1t:
streq.w r5, [r11]
moveq pc, r0
@ %cc1f:
cmp r7, #42
itt ne
@ %cc2t:
strne.w r5, [r8]
movne pc, r10
@ %bb1:
...
rdar://20192768
Differential Revision: http://reviews.llvm.org/D8509
llvm-svn: 232872
diff --git a/llvm/lib/CodeGen/IfConversion.cpp b/llvm/lib/CodeGen/IfConversion.cpp
index 2395f32..b8799a5 100644
--- a/llvm/lib/CodeGen/IfConversion.cpp
+++ b/llvm/lib/CodeGen/IfConversion.cpp
@@ -726,6 +726,12 @@
if (BBI.IsDone || BBI.IsUnpredicable)
return false;
+ // If it is already predicated but we couldn't analyze its terminator, the
+ // latter might fallthrough, but we can't determine where to.
+ // Conservatively avoid if-converting again.
+ if (BBI.Predicate.size() && !BBI.IsBrAnalyzable)
+ return false;
+
// If it is already predicated, check if the new predicate subsumes
// its predicate.
if (BBI.Predicate.size() && !TII->SubsumesPredicate(Pred, BBI.Predicate))