[PowerPC] Mark zext of a small scalar load as free

This initial implementation of PPCTargetLowering::isZExtFree marks as free
zexts of small scalar loads (that are not sign-extending). This callback is
used by SelectionDAGBuilder's RegsForValue::getCopyToRegs, and thus to
determine whether a zext or an anyext is used to lower illegally-typed PHIs.
Because later truncates of zero-extended values are nops, this allows for the
elimination of later unnecessary truncations.

Fixes the initial complaint associated with PR22120.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225584 91177308-0d34-0410-b5e6-96231b3b80d8

3 files changed, 59 insertions, 0 deletions

diff --git a/lib/Target/PowerPC/PPCISelLowering.cpp b/lib/Target/PowerPC/PPCISelLowering.cpp
index 6b24c21c8bc..e93c7819b1a 100644
--- a/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -9780,6 +9780,26 @@ bool PPCTargetLowering::isTruncateFree(EVT VT1, EVT VT2) const {
   return NumBits1 == 64 && NumBits2 == 32;
 }
 
+bool PPCTargetLowering::isZExtFree(SDValue Val, EVT VT2) const {
+  // Generally speaking, zexts are not free, but they are free when they can be
+  // folded with other operations.
+  if (LoadSDNode *LD = dyn_cast<LoadSDNode>(Val)) {
+    EVT MemVT = LD->getMemoryVT();
+    if ((MemVT == MVT::i1 || MemVT == MVT::i8 || MemVT == MVT::i16 ||
+         (Subtarget.isPPC64() && MemVT == MVT::i32)) &&
+        (LD->getExtensionType() == ISD::NON_EXTLOAD ||
+         LD->getExtensionType() == ISD::ZEXTLOAD))
+      return true;
+  }
+
+  // FIXME: Add other cases...
+  //  - 32-bit shifts with a zext to i64
+  //  - zext after ctlz, bswap, etc.
+  //  - zext after and by a constant mask
+
+  return TargetLowering::isZExtFree(Val, VT2);
+}
+
 bool PPCTargetLowering::isLegalICmpImmediate(int64_t Imm) const {
   return isInt<16>(Imm) || isUInt<16>(Imm);
 }
diff --git a/lib/Target/PowerPC/PPCISelLowering.h b/lib/Target/PowerPC/PPCISelLowering.h
index 6149a21f443..db5a3e42d52 100644
--- a/lib/Target/PowerPC/PPCISelLowering.h
+++ b/lib/Target/PowerPC/PPCISelLowering.h
@@ -526,6 +526,8 @@ namespace llvm {
     bool isTruncateFree(Type *Ty1, Type *Ty2) const override;
     bool isTruncateFree(EVT VT1, EVT VT2) const override;
 
+    bool isZExtFree(SDValue Val, EVT VT2) const override;
+
     /// \brief Returns true if it is beneficial to convert a load of a constant
     /// to just the constant itself.
     bool shouldConvertConstantLoadToIntImm(const APInt &Imm,
diff --git a/test/CodeGen/PowerPC/zext-free.ll b/test/CodeGen/PowerPC/zext-free.ll
new file mode 100644
index 00000000000..080dbaa58da
--- /dev/null
+++ b/test/CodeGen/PowerPC/zext-free.ll
@@ -0,0 +1,37 @@
+; RUN: llc -mcpu=ppc64 < %s | FileCheck %s
+target datalayout = "E-m:e-i64:64-n32:64"
+target triple = "powerpc64-unknown-linux-gnu"
+
+; Function Attrs: noreturn nounwind
+define signext i32 @_Z1fRPc(i8** nocapture dereferenceable(8) %p) #0 {
+entry:
+  %.pre = load i8** %p, align 8
+  br label %loop
+
+loop:                                             ; preds = %loop.backedge, %entry
+  %0 = phi i8* [ %.pre, %entry ], [ %.be, %loop.backedge ]
+  %1 = load i8* %0, align 1
+  %tobool = icmp eq i8 %1, 0
+  %incdec.ptr = getelementptr inbounds i8* %0, i64 1
+  store i8* %incdec.ptr, i8** %p, align 8
+  %2 = load i8* %incdec.ptr, align 1
+  %tobool2 = icmp ne i8 %2, 0
+  %or.cond = and i1 %tobool, %tobool2
+  br i1 %or.cond, label %if.then3, label %loop.backedge
+
+if.then3:                                         ; preds = %loop
+  %incdec.ptr4 = getelementptr inbounds i8* %0, i64 2
+  store i8* %incdec.ptr4, i8** %p, align 8
+  br label %loop.backedge
+
+loop.backedge:                                    ; preds = %if.then3, %loop
+  %.be = phi i8* [ %incdec.ptr4, %if.then3 ], [ %incdec.ptr, %loop ]
+  br label %loop
+
+; CHECK-LABEL: @_Z1fRPc
+; CHECK-NOT: rlwinm {{[0-9]+}}, {{[0-9]+}}, 0, 24, 31
+; CHECK-NOT: clrlwi {{[0-9]+}}, {{[0-9]+}}, 24
+}
+
+attributes #0 = { noreturn nounwind }
+