[llvm] [PowerPC] Utilize getReservedRegs to find asm clobberable registers. (PR #107863)

[Amy Kwan via llvm-commits]

================
@@ -380,23 +380,24 @@ BitVector PPCRegisterInfo::getReservedRegs(const MachineFunction &MF) const {
 
   markSuperRegs(Reserved, PPC::VRSAVE);
 
+  const PPCFunctionInfo *FuncInfo = MF.getInfo<PPCFunctionInfo>();
+  bool UsesTOCBasePtr = FuncInfo->usesTOCBasePtr();
   // The SVR4 ABI reserves r2 and r13
   if (Subtarget.isSVR4ABI()) {
     // We only reserve r2 if we need to use the TOC pointer. If we have no
     // explicit uses of the TOC pointer (meaning we're a leaf function with
     // no constant-pool loads, etc.) and we have no potential uses inside an
     // inline asm block, then we can treat r2 has an ordinary callee-saved
     // register.
-    const PPCFunctionInfo *FuncInfo = MF.getInfo<PPCFunctionInfo>();
-    if (!TM.isPPC64() || FuncInfo->usesTOCBasePtr() || MF.hasInlineAsm())
-      markSuperRegs(Reserved, PPC::R2);  // System-reserved register
-    markSuperRegs(Reserved, PPC::R13); // Small Data Area pointer register
+    if (!TM.isPPC64() || UsesTOCBasePtr || MF.hasInlineAsm())
+      markSuperRegs(Reserved, PPC::R2); // System-reserved register.
+    markSuperRegs(Reserved, PPC::R13);  // Small Data Area pointer register.
   }
 
-  // Always reserve r2 on AIX for now.
-  // TODO: Make r2 allocatable on AIX/XCOFF for some leaf functions.
   if (Subtarget.isAIXABI())
-    markSuperRegs(Reserved, PPC::R2);  // System-reserved register
+    // We only reserve r2 if we need to use the TOC pointer on AIX.
+    if (!TM.isPPC64() || UsesTOCBasePtr || MF.hasInlineAsm())
+      markSuperRegs(Reserved, PPC::R2); // System-reserved register.
----------------
amy-kwan wrote:

I think this makes sense. And then the R13 for `isSVR4ABI()` can be handled separately. 

https://github.com/llvm/llvm-project/pull/107863