[PowerPC] Add options for PPC to enable/disable using non-volatile CR

An option is added for PowerPC to disable use of non-volatile CR
register fields and avoid CR spilling in the prologue.

Differential Revision: https://reviews.llvm.org/D69835

lib/Target/PowerPC/PPCMachineFunctionInfo.cpp

@@ -10,10 +10,17 @@
 #include "llvm/ADT/Twine.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/MC/MCContext.h"
+#include "llvm/Support/CommandLine.h"
 
 using namespace llvm;
+static cl::opt<bool> PPCDisableNonVolatileCR(
+    "ppc-disable-non-volatile-cr",
+    cl::desc("Disable the use of non-volatile CR register fields"),
+    cl::init(false), cl::Hidden);
 
 void PPCFunctionInfo::anchor() {}
+PPCFunctionInfo::PPCFunctionInfo(MachineFunction &MF)
+    : DisableNonVolatileCR(PPCDisableNonVolatileCR), MF(MF) {}
 
 MCSymbol *PPCFunctionInfo::getPICOffsetSymbol() const {
   const DataLayout &DL = MF.getDataLayout();

lib/Target/PowerPC/PPCMachineFunctionInfo.h

@@ -65,6 +65,10 @@ class PPCFunctionInfo : public MachineFunctionInfo {
   /// SpillsCR - Indicates whether CR is spilled in the current function.
   bool SpillsCR = false;
 
+  /// DisableNonVolatileCR - Indicates whether non-volatile CR fields would be
+  /// disabled.
+  bool DisableNonVolatileCR = false;
+
   /// Indicates whether VRSAVE is spilled in the current function.
   bool SpillsVRSAVE = false;
 
@@ -129,7 +133,7 @@ class PPCFunctionInfo : public MachineFunctionInfo {
   std::vector<std::pair<unsigned, ISD::ArgFlagsTy>> LiveInAttrs;
 
 public:
-  explicit PPCFunctionInfo(MachineFunction &MF) : MF(MF) {}
+  explicit PPCFunctionInfo(MachineFunction &MF);
 
   int getFramePointerSaveIndex() const { return FramePointerSaveIndex; }
   void setFramePointerSaveIndex(int Idx) { FramePointerSaveIndex = Idx; }
@@ -175,6 +179,9 @@ public:
   void setSpillsCR()       { SpillsCR = true; }
   bool isCRSpilled() const { return SpillsCR; }
 
+  void setDisableNonVolatileCR() { DisableNonVolatileCR = true; }
+  bool isNonVolatileCRDisabled() const { return DisableNonVolatileCR; }
+
   void setSpillsVRSAVE()       { SpillsVRSAVE = true; }
   bool isVRSAVESpilled() const { return SpillsVRSAVE; }
 

lib/Target/PowerPC/PPCRegisterInfo.td

@@ -363,11 +363,23 @@ def CRBITRC : RegisterClass<"PPC", [i1], 32,
        CR1LT, CR1GT, CR1EQ, CR1UN,
        CR0LT, CR0GT, CR0EQ, CR0UN)> {
   let Size = 32;
+  let AltOrders = [(sub CRBITRC, CR2LT, CR2GT, CR2EQ, CR2UN, CR3LT, CR3GT,
+                        CR3EQ, CR3UN, CR4LT, CR4GT, CR4EQ, CR4UN)];
+  let AltOrderSelect = [{
+    return MF.getSubtarget<PPCSubtarget>().isELFv2ABI() &&
+           MF.getInfo<PPCFunctionInfo>()->isNonVolatileCRDisabled();
+  }];
 }
 
-def CRRC : RegisterClass<"PPC", [i32], 32, (add CR0, CR1, CR5, CR6,
-                                                CR7, CR2, CR3, CR4)>;
-
+def CRRC : RegisterClass<"PPC", [i32], 32,
+  (add CR0, CR1, CR5, CR6,
+       CR7, CR2, CR3, CR4)> {
+  let AltOrders = [(sub CRRC, CR2, CR3, CR4)];
+  let AltOrderSelect = [{
+    return MF.getSubtarget<PPCSubtarget>().isELFv2ABI() &&
+           MF.getInfo<PPCFunctionInfo>()->isNonVolatileCRDisabled();
+  }];
+}
 // The CTR registers are not allocatable because they're used by the
 // decrement-and-branch instructions, and thus need to stay live across
 // multiple basic blocks.

test/CodeGen/PowerPC/ppc-disable-non-volatile-cr.ll

@@ -0,0 +1,47 @@
+; Note: Test option to disable use of non-volatile CR to avoid CR spilling in prologue.
+; RUN: llc -verify-machineinstrs -mtriple=powerpc64le-unknown-linux-gnu -ppc-disable-non-volatile-cr\
+; RUN:     -ppc-asm-full-reg-names -ppc-vsr-nums-as-vr < %s | FileCheck --check-prefix=CHECK-DISABLE %s
+; RUN: llc -verify-machineinstrs -mtriple=powerpc64le-unknown-linux-gnu\
+; RUN:     -ppc-asm-full-reg-names -ppc-vsr-nums-as-vr < %s | FileCheck --check-prefix=CHECK-ENABLE %s
+
+; Function Attrs: nounwind
+define dso_local signext i32 @DisableNonVolatileCR(i32 signext %a, i32 signext %b) {
+; CHECK-DISABLE-LABEL: DisableNonVolatileCR:
+; CHECK-DISABLE:       # %bb.0: # %entry
+; CHECK-DISABLE-NOT:    mfocrf [[REG1:r[0-9]+]]
+; CHECK-DISABLE-NOT:    stw [[REG1]]
+; CHECK-DISABLE:        stdu r1
+; CHECK-DISABLE-DAG:    mfocrf [[REG2:r[0-9]+]]
+; CHECK-DISABLE-DAG:    stw [[REG2]]
+; CHECK-DISABLE:        # %bb.1: # %if.then
+;
+; CHECK-ENABLE-LABEL: DisableNonVolatileCR:
+; CHECK-ENABLE:       # %bb.0: # %entry
+; CHECK-ENABLE-DAG:    mfocrf [[REG1:r[0-9]+]]
+; CHECK-ENABLE-DAG:    stw [[REG1]]
+; CHECK-ENABLE:        stdu r1
+; CHECK-ENABLE-NOT:    mfocrf [[REG2:r[0-9]+]]
+; CHECK-ENABLE-NOT:    stw [[REG2]]
+; CHECK-ENABLE:        # %bb.1: # %if.then
+
+entry:
+  %cmp = icmp slt i32 %a, %b
+  br i1 %cmp, label %if.then, label %if.else
+
+if.then:                                          ; preds = %entry
+  tail call void bitcast (void (...)* @fa to void ()*)()
+  br label %if.end
+
+if.else:                                          ; preds = %entry
+  tail call void bitcast (void (...)* @fb to void ()*)()
+  br label %if.end
+
+if.end:                                           ; preds = %if.else, %if.then
+  %conv = zext i1 %cmp to i32
+  %call = tail call signext i32 @callee(i32 signext %conv)
+  ret i32 %call
+}
+
+declare void @fa(...)
+declare void @fb(...)
+declare signext i32 @callee(i32 signext)