From b8dd40fde8aed212ffa8a98b45821ccdf0c2e17d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alex=20R=C3=B8nne=20Petersen?= <alex@alexrp.com>
Date: Tue, 14 Oct 2025 09:39:15 +0200
Subject: [PATCH] std.debug.cpu_context.Sparc: flush register windows in
 current()

It's better to do this here than in StackIterator.init() so that
std.debug.cpu_context.Native.current() isn't a footgun on SPARC.
---
 lib/std/debug.zig             |  2 +-
 lib/std/debug/cpu_context.zig | 11 +++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/lib/std/debug.zig b/lib/std/debug.zig
index 3f694bec1c..8501df175a 100644
--- a/lib/std/debug.zig
+++ b/lib/std/debug.zig
@@ -807,7 +807,6 @@ const StackIterator = union(enum) {
     /// `@frameAddress` and `cpu_context.Native.current` as the caller's stack frame and
     /// our own are one and the same.
     inline fn init(opt_context_ptr: ?CpuContextPtr) error{CannotUnwindFromContext}!StackIterator {
-        flushSparcWindows();
         if (opt_context_ptr) |context_ptr| {
             if (SelfInfo == void or !SelfInfo.can_unwind) return error.CannotUnwindFromContext;
             // Use `di_first` here so we report the PC in the context before unwinding any further.
@@ -826,6 +825,7 @@ const StackIterator = union(enum) {
             // in our caller's frame and above.
             return .{ .di = .init(&.current()) };
         }
+        flushSparcWindows();
         return .{ .fp = @frameAddress() };
     }
     fn deinit(si: *StackIterator) void {
diff --git a/lib/std/debug/cpu_context.zig b/lib/std/debug/cpu_context.zig
index 3450debba0..5dd2e283ed 100644
--- a/lib/std/debug/cpu_context.zig
+++ b/lib/std/debug/cpu_context.zig
@@ -870,6 +870,8 @@ const Sparc = extern struct {
     pub const Gpr = if (native_arch == .sparc64) u64 else u32;
 
     pub inline fn current() Sparc {
+        flushWindows();
+
         var ctx: Sparc = undefined;
         asm volatile (if (Gpr == u64)
                 \\ stx %g0, [%l0 + 0]
@@ -933,6 +935,15 @@ const Sparc = extern struct {
         return ctx;
     }
 
+    noinline fn flushWindows() void {
+        // Flush all register windows except the current one (hence `noinline`). This ensures that
+        // we actually see meaningful data on the stack when we walk the frame chain.
+        if (comptime builtin.target.cpu.has(.sparc, .v9))
+            asm volatile ("flushw" ::: .{ .memory = true })
+        else
+            asm volatile ("ta 3" ::: .{ .memory = true }); // ST_FLUSH_WINDOWS
+    }
+
     pub fn dwarfRegisterBytes(ctx: *Sparc, register_num: u16) DwarfRegisterError![]u8 {
         switch (register_num) {
             0...7 => return @ptrCast(&ctx.g[register_num]),