[PPC, FastISel] Fix ordered/unordered fcmp

For fcmp, major concern about the following 6 cases is NaN result. The
comparison result consists of 4 bits, indicating lt, eq, gt and un (unordered),
only one of which will be set. The result is generated by fcmpu
instruction. However, bc instruction only inspects one of the first 3
bits, so when un is set, bc instruction may jump to to an undesired
place.

More specifically, if we expect an unordered comparison and un is set, we
expect to always go to true branch; in such case UEQ, UGT and ULT still
give false, which are undesired; but UNE, UGE, ULE happen to give true,
since they are tested by inspecting !eq, !lt, !gt, respectively.

Similarly, for ordered comparison, when un is set, we always expect the
result to be false. In such case OGT, OLT and OEQ is good, since they are
actually testing GT, LT, and EQ respectively, which are false. OGE, OLE
and ONE are tested through !lt, !gt and !eq, and these are true.

llvm-svn: 263753

1 files changed, 23 insertions, 7 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCFastISel.cpp b/llvm/lib/Target/PowerPC/PPCFastISel.cpp
index 1de26790651..360fbb61bc5 100644
--- a/llvm/lib/Target/PowerPC/PPCFastISel.cpp
+++ b/llvm/lib/Target/PowerPC/PPCFastISel.cpp
@@ -213,13 +213,29 @@ static Optional<PPC::Predicate> getComparePred(CmpInst::Predicate Pred) {
   switch (Pred) {
     // These are not representable with any single compare.
     case CmpInst::FCMP_FALSE:
+    case CmpInst::FCMP_TRUE:
+    // Major concern about the following 6 cases is NaN result. The comparison
+    // result consists of 4 bits, indicating lt, eq, gt and un (unordered),
+    // only one of which will be set. The result is generated by fcmpu
+    // instruction. However, bc instruction only inspects one of the first 3
+    // bits, so when un is set, bc instruction may jump to to an undesired
+    // place.
+    //
+    // More specifically, if we expect an unordered comparison and un is set, we
+    // expect to always go to true branch; in such case UEQ, UGT and ULT still
+    // give false, which are undesired; but UNE, UGE, ULE happen to give true,
+    // since they are tested by inspecting !eq, !lt, !gt, respectively.
+    //
+    // Similarly, for ordered comparison, when un is set, we always expect the
+    // result to be false. In such case OGT, OLT and OEQ is good, since they are
+    // actually testing GT, LT, and EQ respectively, which are false. OGE, OLE
+    // and ONE are tested through !lt, !gt and !eq, and these are true.
     case CmpInst::FCMP_UEQ:
     case CmpInst::FCMP_UGT:
-    case CmpInst::FCMP_UGE:
     case CmpInst::FCMP_ULT:
-    case CmpInst::FCMP_ULE:
-    case CmpInst::FCMP_UNE:
-    case CmpInst::FCMP_TRUE:
+    case CmpInst::FCMP_OGE:
+    case CmpInst::FCMP_OLE:
+    case CmpInst::FCMP_ONE:
     default:
       return Optional<PPC::Predicate>();
 
@@ -232,7 +248,7 @@ static Optional<PPC::Predicate> getComparePred(CmpInst::Predicate Pred) {
     case CmpInst::ICMP_SGT:
       return PPC::PRED_GT;
 
-    case CmpInst::FCMP_OGE:
+    case CmpInst::FCMP_UGE:
     case CmpInst::ICMP_UGE:
     case CmpInst::ICMP_SGE:
       return PPC::PRED_GE;
@@ -242,12 +258,12 @@ static Optional<PPC::Predicate> getComparePred(CmpInst::Predicate Pred) {
     case CmpInst::ICMP_SLT:
       return PPC::PRED_LT;
 
-    case CmpInst::FCMP_OLE:
+    case CmpInst::FCMP_ULE:
     case CmpInst::ICMP_ULE:
     case CmpInst::ICMP_SLE:
       return PPC::PRED_LE;
 
-    case CmpInst::FCMP_ONE:
+    case CmpInst::FCMP_UNE:
     case CmpInst::ICMP_NE:
       return PPC::PRED_NE;