[PowerPC] Handle cmp op promotion for SELECT[_CC] nodes in PPCTL::DAGCombineExtBoolTrunc

PPCTargetLowering::DAGCombineExtBoolTrunc contains logic to remove unwanted
truncations and extensions when dealing with nodes of the form:
  zext(binary-ops(binary-ops(trunc(x), trunc(y)), ...)

There was a FIXME in the implementation (now removed) regarding the fact that
the function would abort the transformations if any of the non-output operands
of a SELECT or SELECT_CC node would need to be promoted (because they were
also output operands, for example). As a result, we continued to generate
unnecessary zero-extends for code such as this:

  unsigned foo(unsigned a, unsigned b) {
    return  (a <= b) ? a : b;
  }

which would produce:

  cmplw 0, 3, 4
  isel 3, 4, 3, 1
  rldicl 3, 3, 0, 32
  blr

and now we produce:

  cmplw 0, 3, 4
  isel 3, 4, 3, 1
  blr

which is better in the obvious way.

llvm-svn: 224213

2 files changed, 60 insertions, 18 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
index 29bbad4808b..62c1f714e9d 100644
--- a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -8127,6 +8127,10 @@ SDValue PPCTargetLowering::DAGCombineExtBoolTrunc(SDNode *N,
     }
   }
 
+  // The operands of a select that must be truncated when the select is
+  // promoted because the operand is actually part of the to-be-promoted set.
+  DenseMap<SDNode *, EVT> SelectTruncOp[2];
+
   // Make sure that this is a self-contained cluster of operations (which
   // is not quite the same thing as saying that everything has only one
   // use).
@@ -8141,18 +8145,19 @@ SDValue PPCTargetLowering::DAGCombineExtBoolTrunc(SDNode *N,
       if (User != N && !Visited.count(User))
         return SDValue();
 
-      // Make sure that we're not going to promote the non-output-value
-      // operand(s) or SELECT or SELECT_CC.
-      // FIXME: Although we could sometimes handle this, and it does occur in
-      // practice that one of the condition inputs to the select is also one of
-      // the outputs, we currently can't deal with this.
+      // If we're going to promote the non-output-value operand(s) or SELECT or
+      // SELECT_CC, record them for truncation.
       if (User->getOpcode() == ISD::SELECT) {
         if (User->getOperand(0) == Inputs[i])
-          return SDValue();
+          SelectTruncOp[0].insert(std::make_pair(User,
+                                    User->getOperand(0).getValueType()));
       } else if (User->getOpcode() == ISD::SELECT_CC) {
-        if (User->getOperand(0) == Inputs[i] ||
-            User->getOperand(1) == Inputs[i])
-          return SDValue();
+        if (User->getOperand(0) == Inputs[i])
+          SelectTruncOp[0].insert(std::make_pair(User,
+                                    User->getOperand(0).getValueType()));
+        if (User->getOperand(1) == Inputs[i])
+          SelectTruncOp[1].insert(std::make_pair(User,
+                                    User->getOperand(1).getValueType()));
       }
     }
   }
@@ -8165,18 +8170,19 @@ SDValue PPCTargetLowering::DAGCombineExtBoolTrunc(SDNode *N,
       if (User != N && !Visited.count(User))
         return SDValue();
 
-      // Make sure that we're not going to promote the non-output-value
-      // operand(s) or SELECT or SELECT_CC.
-      // FIXME: Although we could sometimes handle this, and it does occur in
-      // practice that one of the condition inputs to the select is also one of
-      // the outputs, we currently can't deal with this.
+      // If we're going to promote the non-output-value operand(s) or SELECT or
+      // SELECT_CC, record them for truncation.
       if (User->getOpcode() == ISD::SELECT) {
         if (User->getOperand(0) == PromOps[i])
-          return SDValue();
+          SelectTruncOp[0].insert(std::make_pair(User,
+                                    User->getOperand(0).getValueType()));
       } else if (User->getOpcode() == ISD::SELECT_CC) {
-        if (User->getOperand(0) == PromOps[i] ||
-            User->getOperand(1) == PromOps[i])
-          return SDValue();
+        if (User->getOperand(0) == PromOps[i])
+          SelectTruncOp[0].insert(std::make_pair(User,
+                                    User->getOperand(0).getValueType()));
+        if (User->getOperand(1) == PromOps[i])
+          SelectTruncOp[1].insert(std::make_pair(User,
+                                    User->getOperand(1).getValueType()));
       }
     }
   }
@@ -8257,6 +8263,19 @@ SDValue PPCTargetLowering::DAGCombineExtBoolTrunc(SDNode *N,
       continue;
     }
 
+    // For SELECT and SELECT_CC nodes, we do a similar check for any
+    // to-be-promoted comparison inputs.
+    if (PromOp.getOpcode() == ISD::SELECT ||
+        PromOp.getOpcode() == ISD::SELECT_CC) {
+      if ((SelectTruncOp[0].count(PromOp.getNode()) &&
+           PromOp.getOperand(0).getValueType() != N->getValueType(0)) ||
+          (SelectTruncOp[1].count(PromOp.getNode()) &&
+           PromOp.getOperand(1).getValueType() != N->getValueType(0))) {
+        PromOps.insert(PromOps.begin(), PromOp);
+        continue;
+      }
+    }
+
     SmallVector<SDValue, 3> Ops(PromOp.getNode()->op_begin(),
                                 PromOp.getNode()->op_end());
 
@@ -8275,6 +8294,18 @@ SDValue PPCTargetLowering::DAGCombineExtBoolTrunc(SDNode *N,
         Ops[C+i] = DAG.getAnyExtOrTrunc(Ops[C+i], dl, N->getValueType(0));
     }
 
+    // If we've promoted the comparison inputs of a SELECT or SELECT_CC,
+    // truncate them again to the original value type.
+    if (PromOp.getOpcode() == ISD::SELECT ||
+        PromOp.getOpcode() == ISD::SELECT_CC) {
+      auto SI0 = SelectTruncOp[0].find(PromOp.getNode());
+      if (SI0 != SelectTruncOp[0].end())
+        Ops[0] = DAG.getNode(ISD::TRUNCATE, dl, SI0->second, Ops[0]);
+      auto SI1 = SelectTruncOp[1].find(PromOp.getNode());
+      if (SI1 != SelectTruncOp[1].end())
+        Ops[1] = DAG.getNode(ISD::TRUNCATE, dl, SI1->second, Ops[1]);
+    }
+
     DAG.ReplaceAllUsesOfValueWith(PromOp,
       DAG.getNode(PromOp.getOpcode(), dl, N->getValueType(0), Ops));
   }
diff --git a/llvm/test/CodeGen/PowerPC/rm-zext.ll b/llvm/test/CodeGen/PowerPC/rm-zext.ll
index 1e60d1198bc..8fab38ad4cf 100644
--- a/llvm/test/CodeGen/PowerPC/rm-zext.ll
+++ b/llvm/test/CodeGen/PowerPC/rm-zext.ll
@@ -28,5 +28,16 @@ entry:
 ; CHECK: blr
 }
 
+define zeroext i32 @min(i32 zeroext %a, i32 zeroext %b) #0 {
+entry:
+  %cmp = icmp ule i32 %a, %b
+  %cond = select i1 %cmp, i32 %a, i32 %b
+  ret i32 %cond
+
+; CHECK-LABEL @min
+; CHECK-NOT: rldicl 3, {{[0-9]+}}, 0, 32
+; CHECK: blr
+}
+
 attributes #0 = { nounwind readnone }