[CallingConv] Return ArrayRef from AllocateRegBlock() (NFC) (#124120)

Instead of returning the first register, return the ArrayRef containing
the whole block.

Existing users rely on the fact that the register block only contains
adjacently-numbered registers and it's possible to get the remaining
registers in the block by just incrementing the register. Returning an
ArrayRef allows more generic usage with non-adjacent registers.

llvm/include/llvm/CodeGen/CallingConvLower.h

@@ -357,12 +357,13 @@ class CCState {
     return Reg;
   }
 
-  /// AllocateRegBlock - Attempt to allocate a block of RegsRequired consecutive
-  /// registers. If this is not possible, return zero. Otherwise, return the first
-  /// register of the block that were allocated, marking the entire block as allocated.
-  MCPhysReg AllocateRegBlock(ArrayRef<MCPhysReg> Regs, unsigned RegsRequired) {
+  /// Attempt to allocate a block of RegsRequired consecutive registers.
+  /// If this is not possible, return an empty range. Otherwise, return a
+  /// range of consecutive registers, marking the entire block as allocated.
+  ArrayRef<MCPhysReg> AllocateRegBlock(ArrayRef<MCPhysReg> Regs,
+                                       unsigned RegsRequired) {
     if (RegsRequired > Regs.size())
-      return 0;
+      return {};
 
     for (unsigned StartIdx = 0; StartIdx <= Regs.size() - RegsRequired;
          ++StartIdx) {
@@ -379,11 +380,11 @@ class CCState {
         for (unsigned BlockIdx = 0; BlockIdx < RegsRequired; ++BlockIdx) {
           MarkAllocated(Regs[StartIdx + BlockIdx]);
         }
-        return Regs[StartIdx];
+        return Regs.slice(StartIdx, RegsRequired);
       }
     }
     // No block was available
-    return 0;
+    return {};
   }
 
   /// Version of AllocateReg with list of registers to be shadowed.

llvm/lib/Target/AArch64/AArch64CallingConvention.cpp

@@ -176,27 +176,27 @@ static bool CC_AArch64_Custom_Block(unsigned &ValNo, MVT &ValVT, MVT &LocVT,
   // [N x i32] arguments get packed into x-registers on Darwin's arm64_32
   // because that's how the armv7k Clang front-end emits small structs.
   unsigned EltsPerReg = (IsDarwinILP32 && LocVT.SimpleTy == MVT::i32) ? 2 : 1;
-  unsigned RegResult = State.AllocateRegBlock(
+  ArrayRef<MCPhysReg> RegResult = State.AllocateRegBlock(
       RegList, alignTo(PendingMembers.size(), EltsPerReg) / EltsPerReg);
-  if (RegResult && EltsPerReg == 1) {
-    for (auto &It : PendingMembers) {
-      It.convertToReg(RegResult);
+  if (!RegResult.empty() && EltsPerReg == 1) {
+    for (const auto &[It, Reg] : zip(PendingMembers, RegResult)) {
+      It.convertToReg(Reg);
       State.addLoc(It);
-      ++RegResult;
     }
     PendingMembers.clear();
     return true;
-  } else if (RegResult) {
+  } else if (!RegResult.empty()) {
     assert(EltsPerReg == 2 && "unexpected ABI");
     bool UseHigh = false;
     CCValAssign::LocInfo Info;
+    unsigned RegIdx = 0;
     for (auto &It : PendingMembers) {
       Info = UseHigh ? CCValAssign::AExtUpper : CCValAssign::ZExt;
-      State.addLoc(CCValAssign::getReg(It.getValNo(), MVT::i32, RegResult,
-                                       MVT::i64, Info));
+      State.addLoc(CCValAssign::getReg(It.getValNo(), MVT::i32,
+                                       RegResult[RegIdx], MVT::i64, Info));
       UseHigh = !UseHigh;
       if (!UseHigh)
-        ++RegResult;
+        ++RegIdx;
     }
     PendingMembers.clear();
     return true;

llvm/lib/Target/ARM/ARMCallingConv.cpp

@@ -228,12 +228,12 @@ static bool CC_ARM_AAPCS_Custom_Aggregate(unsigned ValNo, MVT ValVT,
     break;
   }
 
-  unsigned RegResult = State.AllocateRegBlock(RegList, PendingMembers.size());
-  if (RegResult) {
-    for (CCValAssign &PendingMember : PendingMembers) {
-      PendingMember.convertToReg(RegResult);
+  ArrayRef<MCPhysReg> RegResult =
+      State.AllocateRegBlock(RegList, PendingMembers.size());
+  if (!RegResult.empty()) {
+    for (const auto &[PendingMember, Reg] : zip(PendingMembers, RegResult)) {
+      PendingMember.convertToReg(Reg);
       State.addLoc(PendingMember);
-      ++RegResult;
     }
     PendingMembers.clear();
     return true;