[llvm] [X86] Use generic CPU tuning when tune-cpu is empty (PR #83631)

[via llvm-commits]

https://github.com/AtariDreams updated https://github.com/llvm/llvm-project/pull/83631

>From 1f7d95605916f8f3a19c843f47b844869c3f0fe0 Mon Sep 17 00:00:00 2001
From: Rose <83477269+AtariDreams at users.noreply.github.com>
Date: Fri, 1 Mar 2024 17:52:58 -0500
Subject: [PATCH] [X86] Use generic CPU tuning when tune-cpu is empty

To prevent the test results from being bugged, I ensured this change did not affect 32-bit targets.
---
 llvm/lib/Target/X86/X86Subtarget.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/llvm/lib/Target/X86/X86Subtarget.cpp b/llvm/lib/Target/X86/X86Subtarget.cpp
index 07f535685e8f97..1372b1bfc6f079 100644
--- a/llvm/lib/Target/X86/X86Subtarget.cpp
+++ b/llvm/lib/Target/X86/X86Subtarget.cpp
@@ -252,7 +252,7 @@ void X86Subtarget::initSubtargetFeatures(StringRef CPU, StringRef TuneCPU,
     CPU = "generic";
 
   if (TuneCPU.empty())
-    TuneCPU = "i586"; // FIXME: "generic" is more modern than llc tests expect.
+    TuneCPU = Is64Bit ? "x86-64" : "i586";
 
   std::string FullFS = X86_MC::ParseX86Triple(TargetTriple);
   assert(!FullFS.empty() && "Failed to parse X86 triple");