[libc-commits] [libc] [libc][AArch64] Support LFI in setjmp and longjmp (PR #238092)

[Christian Nieves via libc-commits]

https://github.com/squk updated https://github.com/llvm/llvm-project/pull/238092

>From 865b37a58a205841ca7a2438c8bb82c0cd91c437 Mon Sep 17 00:00:00 2001
From: Christian Nieves <christian at nieves.io>
Date: Mon, 2 Nov 2026 20:38:14 +0000
Subject: [PATCH] [libc][AArch64] Support LFI in setjmp and longjmp

Skip saving and restoring LFI-reserved registers (x25, x27, and x28) in
AArch64 setjmp and longjmp when building for LFI (__LFI__).
---
 .../__support/macros/properties/architectures.h   |  4 ++++
 libc/src/setjmp/aarch64/longjmp.cpp               | 15 ++++++++++++++-
 libc/src/setjmp/aarch64/setjmp.cpp                | 12 ++++++++++++
 3 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/libc/src/__support/macros/properties/architectures.h b/libc/src/__support/macros/properties/architectures.h
index c1c425067ddd9d..18700b66f9a0e6 100644
--- a/libc/src/__support/macros/properties/architectures.h
+++ b/libc/src/__support/macros/properties/architectures.h
@@ -74,4 +74,8 @@
 #define LIBC_TARGET_ARCH_IS_HEXAGON
 #endif
 
+#if defined(__LFI__)
+#define LIBC_TARGET_ARCH_IS_LFI
+#endif
+
 #endif // LLVM_LIBC_SRC___SUPPORT_MACROS_PROPERTIES_ARCHITECTURES_H
diff --git a/libc/src/setjmp/aarch64/longjmp.cpp b/libc/src/setjmp/aarch64/longjmp.cpp
index 80c97c721cc4f6..144524f675c7d3 100644
--- a/libc/src/setjmp/aarch64/longjmp.cpp
+++ b/libc/src/setjmp/aarch64/longjmp.cpp
@@ -9,6 +9,7 @@
 #include "src/setjmp/longjmp.h"
 #include "src/__support/common.h"
 #include "src/__support/macros/config.h"
+#include "src/__support/macros/properties/architectures.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
@@ -32,7 +33,18 @@ namespace LIBC_NAMESPACE_DECL {
   // address at the start of longjmp, because we're not going to use it anyway!
 
   asm(
-      // Reload the callee-saved GPRs, including fp and lr.
+  // Reload the callee-saved GPRs, including fp and lr.
+#if defined(LIBC_TARGET_ARCH_IS_LFI)
+      // LFI reserves x25 (ctx), x27 (membase), and x28 (retbase); also mask sp
+      // and x30 via the LFI target rewriting rules.
+      R"(
+        ldp x19, x20, [x0,  #0*16]
+        ldp x21, x22, [x0,  #1*16]
+        ldp x23, x24, [x0,  #2*16]
+        ldr x26,      [x0,  #3*16 + 8]
+        ldp x29, x30, [x0,  #5*16]
+      )"
+#else
       R"(
         ldp x19, x20, [x0,  #0*16]
         ldp x21, x22, [x0,  #1*16]
@@ -41,6 +53,7 @@ namespace LIBC_NAMESPACE_DECL {
         ldp x27, x28, [x0,  #4*16]
         ldp x29, x30, [x0,  #5*16]
       )"
+#endif
 
 #if LIBC_COPT_SETJMP_AARCH64_RESTORE_PLATFORM_REGISTER
       // Reload the stack pointer, and the platform register x18.
diff --git a/libc/src/setjmp/aarch64/setjmp.cpp b/libc/src/setjmp/aarch64/setjmp.cpp
index 8dd1eb342c0973..9378f54f0a8d53 100644
--- a/libc/src/setjmp/aarch64/setjmp.cpp
+++ b/libc/src/setjmp/aarch64/setjmp.cpp
@@ -8,6 +8,7 @@
 
 #include "src/__support/common.h"
 #include "src/__support/macros/config.h"
+#include "src/__support/macros/properties/architectures.h"
 #include "src/setjmp/setjmp_impl.h"
 
 namespace LIBC_NAMESPACE_DECL {
@@ -33,6 +34,16 @@ namespace LIBC_NAMESPACE_DECL {
       // Of course lr isn't normally callee-saved (the call instruction itself
       // can't help clobbering it), but we certainly need to save it for this
       // purpose.
+#if defined(LIBC_TARGET_ARCH_IS_LFI)
+      // LFI reserves x25 (ctx), x27 (membase), and x28 (retbase).
+      R"(
+        stp x19, x20, [x0,  #0*16]
+        stp x21, x22, [x0,  #1*16]
+        stp x23, x24, [x0,  #2*16]
+        str x26,      [x0,  #3*16 + 8]
+        stp x29, x30, [x0,  #5*16]
+      )"
+#else
       R"(
         stp x19, x20, [x0,  #0*16]
         stp x21, x22, [x0,  #1*16]
@@ -41,6 +52,7 @@ namespace LIBC_NAMESPACE_DECL {
         stp x27, x28, [x0,  #4*16]
         stp x29, x30, [x0,  #5*16]
       )"
+#endif
 
 #if LIBC_COPT_SETJMP_AARCH64_RESTORE_PLATFORM_REGISTER
       // Store the stack pointer, and the platform register x18.