[compiler-rt] [sanitizer_common] Disable the fast unwinder on Alpha Linux (PR #220231)

[Matt Turner via llvm-commits]

https://github.com/mattst88 updated https://github.com/llvm/llvm-project/pull/220231

>From 28cdf392e06290659750703e157b92abbd23a17c Mon Sep 17 00:00:00 2001
From: Matt Turner <mattst88 at gmail.com>
Date: Tue, 1 Sep 2026 08:11:42 -0400
Subject: [PATCH] [sanitizer_common] Disable the fast unwinder on Alpha Linux

BufferedStackTrace::UnwindFast walks the stack assuming frame[0] is the
caller's frame pointer and frame[1] the return address. Alpha chains
neither, so the walk yields a few garbage frames and then stops:

  freed by thread T0 here:
      #0 0x020000971274 in free asan_malloc_linux.cpp:51
      #1 0x0200007fa48c  (<unknown module>)

Opt Alpha out of the fast unwinder the way Linux/MIPS already is. The
_Unwind_Backtrace based unwinder gives complete traces; with it, GCC's
g++.dg/asan/deep-stack-uaf-1.C passes on alpha-unknown-linux-gnu.

Assisted-by: Claude Code
---
 compiler-rt/lib/sanitizer_common/sanitizer_stacktrace.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_stacktrace.h b/compiler-rt/lib/sanitizer_common/sanitizer_stacktrace.h
index 47aed488c71a7..3aea61ba2dc8c 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_stacktrace.h
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_stacktrace.h
@@ -22,12 +22,12 @@ struct BufferedStackTrace;
 
 static const u32 kStackTraceMax = 255;
 
-#if SANITIZER_LINUX && defined(__mips__)
-# define SANITIZER_CAN_FAST_UNWIND 0
+#if SANITIZER_LINUX && (defined(__mips__) || defined(__alpha__))
+#  define SANITIZER_CAN_FAST_UNWIND 0
 #elif SANITIZER_WINDOWS
-# define SANITIZER_CAN_FAST_UNWIND 0
+#  define SANITIZER_CAN_FAST_UNWIND 0
 #else
-# define SANITIZER_CAN_FAST_UNWIND 1
+#  define SANITIZER_CAN_FAST_UNWIND 1
 #endif
 
 // Fast unwind is the only option on Mac for now; we will need to