[arm64] Cleanup TODO around handling of x18

Use `padreg` instead of x18 to maintain alignment in the CPURegList.

Also clean up some comments and tidy up RequiredStackSizeForCallerSaved
and PushCallerSaved.

Change-Id: I80a780e5649e69a1746c43f37c2d1d875120c7a0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1581609Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#60987}

src/arm64/assembler-arm64.cc

@@ -98,6 +98,18 @@ void CPURegList::RemoveCalleeSaved() {
   }
 }
 
+void CPURegList::Align() {
+  // Use padreg, if necessary, to maintain stack alignment.
+  if (Count() % 2 != 0) {
+    if (IncludesAliasOf(padreg)) {
+      Remove(padreg);
+    } else {
+      Combine(padreg);
+    }
+  }
+
+  DCHECK_EQ(Count() % 2, 0);
+}
 
 CPURegList CPURegList::GetCalleeSaved(int size) {
   return CPURegList(CPURegister::kRegister, size, 19, 29);

src/arm64/deoptimizer-arm64.cc

@@ -55,12 +55,6 @@ void CopyRegListToFrame(MacroAssembler* masm, const Register& dst,
   masm->Sub(dst, dst, dst_offset);
 }
 
-// TODO(jgruber): There's a hack here to explicitly skip restoration of the
-// so-called 'arm64 platform register' x18. The register may be in use by the
-// OS, thus we should not clobber it. Instead of this hack, it would be nicer
-// not to add x18 to the list of saved registers in the first place. The
-// complication here is that we require `reg_list.Count() % 2 == 0` in multiple
-// spots.
 void RestoreRegList(MacroAssembler* masm, const CPURegList& reg_list,
                     const Register& src_base, int src_offset) {
   DCHECK_EQ(reg_list.Count() % 2, 0);
@@ -74,8 +68,8 @@ void RestoreRegList(MacroAssembler* masm, const CPURegList& reg_list,
   Register src = temps.AcquireX();
   masm->Add(src, src_base, src_offset);
 
-  // x18 is the platform register and is reserved for the use of platform ABIs.
-  restore_list.Remove(x18);
+  // No need to restore padreg.
+  restore_list.Remove(padreg);
 
   // Restore every register in restore_list from src.
   while (!restore_list.IsEmpty()) {

src/arm64/macro-assembler-arm64.cc

@@ -40,25 +40,11 @@ CPURegList TurboAssembler::DefaultFPTmpList() {
 
 int TurboAssembler::RequiredStackSizeForCallerSaved(SaveFPRegsMode fp_mode,
                                                     Register exclusion) const {
-  int bytes = 0;
   auto list = kCallerSaved;
-  // We only allow one exclusion register, so if the list is of even length
-  // before exclusions, it must still be afterwards, to maintain alignment.
-  // Therefore, we can ignore the exclusion register in the computation.
-  // However, we leave it in the argument list to mirror the prototype for
-  // Push/PopCallerSaved().
-
-  // X18 is excluded from caller-saved register list on ARM64 which makes
-  // caller-saved registers in odd number. padreg is used accordingly to
-  // maintain the alignment.
-  DCHECK_EQ(list.Count() % 2, 1);
-  if (exclusion.Is(no_reg)) {
-    bytes += kXRegSizeInBits / 8;
-  } else {
-    bytes -= kXRegSizeInBits / 8;
-  }
+  list.Remove(exclusion);
+  list.Align();
 
-  bytes += list.Count() * kXRegSizeInBits / 8;
+  int bytes = list.Count() * kXRegSizeInBits / 8;
 
   if (fp_mode == kSaveFPRegs) {
     DCHECK_EQ(kCallerSavedV.Count() % 2, 0);
@@ -69,20 +55,13 @@ int TurboAssembler::RequiredStackSizeForCallerSaved(SaveFPRegsMode fp_mode,
 
 int TurboAssembler::PushCallerSaved(SaveFPRegsMode fp_mode,
                                     Register exclusion) {
-  int bytes = 0;
   auto list = kCallerSaved;
+  list.Remove(exclusion);
+  list.Align();
 
-  // X18 is excluded from caller-saved register list on ARM64, use padreg
-  // accordingly to maintain alignment.
-  if (!exclusion.Is(no_reg)) {
-    list.Remove(exclusion);
-  } else {
-    list.Combine(padreg);
-  }
-
-  DCHECK_EQ(list.Count() % 2, 0);
   PushCPURegList(list);
-  bytes += list.Count() * kXRegSizeInBits / 8;
+
+  int bytes = list.Count() * kXRegSizeInBits / 8;
 
   if (fp_mode == kSaveFPRegs) {
     DCHECK_EQ(kCallerSavedV.Count() % 2, 0);
@@ -101,16 +80,9 @@ int TurboAssembler::PopCallerSaved(SaveFPRegsMode fp_mode, Register exclusion) {
   }
 
   auto list = kCallerSaved;
+  list.Remove(exclusion);
+  list.Align();
 
-  // X18 is excluded from caller-saved register list on ARM64, use padreg
-  // accordingly to maintain alignment.
-  if (!exclusion.Is(no_reg)) {
-    list.Remove(exclusion);
-  } else {
-    list.Combine(padreg);
-  }
-
-  DCHECK_EQ(list.Count() % 2, 0);
   PopCPURegList(list);
   bytes += list.Count() * kXRegSizeInBits / 8;
 
@@ -3378,11 +3350,8 @@ void MacroAssembler::Printf(const char * format,
   TmpList()->set_list(0);
   FPTmpList()->set_list(0);
 
-  // x18 is the platform register and is reserved for the use of platform ABIs.
-  // It is not part of the kCallerSaved list, but we add it here anyway to
-  // ensure `reg_list.Count() % 2 == 0` which is required in multiple spots.
   CPURegList saved_registers = kCallerSaved;
-  saved_registers.Combine(x18.code());
+  saved_registers.Align();
 
   // Preserve all caller-saved registers as well as NZCV.
   // PushCPURegList asserts that the size of each list is a multiple of 16

src/arm64/register-arm64.h

@@ -626,6 +626,9 @@ class V8_EXPORT_PRIVATE CPURegList {
   // preparing registers for an AAPCS64 function call, for example.
   void RemoveCalleeSaved();
 
+  // Align the list to 16 bytes.
+  void Align();
+
   CPURegister PopLowestIndex();
   CPURegister PopHighestIndex();