[llvm-branch-commits] [llvm] [AArch64] Reserve x18 as the platform register on UEFI (PR #232701)

[Prabhu Rajasekaran via llvm-branch-commits]

https://github.com/Prabhuk created https://github.com/llvm/llvm-project/pull/232701

Both the Windows ARM64 ABI, which UEFI follows, and the firmware
ecosystem (EDK2 builds AArch64 with -ffixed-x18) treat x18 as a reserved
platform register. Reserve it by default for aarch64-unknown-uefi, as is
already done for Windows, Darwin, Android, Fuchsia and OHOS.

Assisted-by: Claude

---

<sub>Stack created with <a href="https://github.com/github/gh-stack">GitHub Stacks CLI</a> • <a href="https://gh.io/stacks-feedback">Give Feedback 💬</a></sub>

>From 59524d8d91fc0a13df9e5d0f34def1b07682ae96 Mon Sep 17 00:00:00 2001
From: prabhukr <prabhukr at google.com>
Date: Fri, 16 Oct 2026 04:47:23 -0700
Subject: [PATCH] [AArch64] Reserve x18 as the platform register on UEFI

Both the Windows ARM64 ABI, which UEFI follows, and the firmware
ecosystem (EDK2 builds AArch64 with -ffixed-x18) treat x18 as a reserved
platform register. Reserve it by default for aarch64-unknown-uefi, as is
already done for Windows, Darwin, Android, Fuchsia and OHOS.

Assisted-by: Claude
---
 llvm/lib/TargetParser/AArch64TargetParser.cpp   | 2 +-
 llvm/test/CodeGen/AArch64/arm64-platform-reg.ll | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/TargetParser/AArch64TargetParser.cpp b/llvm/lib/TargetParser/AArch64TargetParser.cpp
index 3624b04ffb81e9..476309717bee63 100644
--- a/llvm/lib/TargetParser/AArch64TargetParser.cpp
+++ b/llvm/lib/TargetParser/AArch64TargetParser.cpp
@@ -145,7 +145,7 @@ void AArch64::fillValidCPUArchList(SmallVectorImpl<StringRef> &Values) {
 
 bool AArch64::isX18ReservedByDefault(const Triple &TT) {
   return TT.isAndroid() || TT.isOSDarwin() || TT.isOSFuchsia() ||
-         TT.isOSWindows() || TT.isOHOSFamily();
+         TT.isOSWindowsOrUEFI() || TT.isOHOSFamily();
 }
 
 // Allows partial match, ex. "v8a" matches "armv8a".
diff --git a/llvm/test/CodeGen/AArch64/arm64-platform-reg.ll b/llvm/test/CodeGen/AArch64/arm64-platform-reg.ll
index 45bb1c69b92132..03a9eb8dc1572d 100644
--- a/llvm/test/CodeGen/AArch64/arm64-platform-reg.ll
+++ b/llvm/test/CodeGen/AArch64/arm64-platform-reg.ll
@@ -5,6 +5,7 @@
 ; RUN: llc -mtriple=aarch64-linux-ohos -o - %s | FileCheck %s --check-prefix=CHECK-RESERVE --check-prefix=CHECK-RESERVE-X18
 ; RUN: llc -mtriple=aarch64-fuchsia -o - %s | FileCheck %s --check-prefix=CHECK-RESERVE --check-prefix=CHECK-RESERVE-X18
 ; RUN: llc -mtriple=aarch64-windows -o - %s | FileCheck %s --check-prefix=CHECK-RESERVE --check-prefix=CHECK-RESERVE-X18
+; RUN: llc -mtriple=aarch64-unknown-uefi -o - %s | FileCheck %s --check-prefix=CHECK-RESERVE --check-prefix=CHECK-RESERVE-X18
 
 ; Test reserve-x# options individually.
 ; RUN: llc -mtriple=arm64-linux-gnu -mattr=+reserve-x1 -o - %s | FileCheck %s --check-prefixes=CHECK-RESERVE,CHECK-RESERVE-X1