From 2040f9bfd0ab874c6b7a748b9293ad31c7392459 Mon Sep 17 00:00:00 2001
From: Jacob Young <jacobly0@users.noreply.github.com>
Date: Sat, 25 Apr 2026 11:59:21 -0400
Subject: [PATCH] x86_64: fix c abi of `f32` struct field followed by padding

Closes #31864
---
 src/codegen/x86_64/CodeGen.zig |  8 +++++---
 src/codegen/x86_64/abi.zig     | 21 +++++++++++++--------
 test/c_abi/cfuncs.c            |  2 --
 test/c_abi/main.zig            | 23 +++++++----------------
 4 files changed, 25 insertions(+), 29 deletions(-)

diff --git a/src/codegen/x86_64/CodeGen.zig b/src/codegen/x86_64/CodeGen.zig
index 72a382fee1..97cee13bb3 100644
--- a/src/codegen/x86_64/CodeGen.zig
+++ b/src/codegen/x86_64/CodeGen.zig
@@ -181632,9 +181632,11 @@ fn splitType(self: *CodeGen, comptime parts_len: usize, ty: Type) ![parts_len]Ty
             else => break,
         };
     } else {
-        var part_sizes: u64 = 0;
-        for (parts) |part| part_sizes += part.abiSize(zcu);
-        if (part_sizes == ty.abiSize(zcu)) return parts;
+        var parts_size: u64 = 0;
+        for (parts) |part| parts_size += part.abiSize(zcu);
+        const abi_size = ty.abiSize(zcu);
+        if (abi_size == parts_size) return parts;
+        if (classes[classes.len - 1] == .float and abi_size > parts_size and abi_size <= parts_size + 4) return parts;
     };
     return self.fail("TODO implement splitType({d}, {f})", .{ parts_len, ty.fmt(pt) });
 }
diff --git a/src/codegen/x86_64/abi.zig b/src/codegen/x86_64/abi.zig
index 911609b1f3..fcd6e63f92 100644
--- a/src/codegen/x86_64/abi.zig
+++ b/src/codegen/x86_64/abi.zig
@@ -300,21 +300,26 @@ pub fn classifySystemV(ty: Type, zcu: *Zcu, target: *const std.Target, ctx: Cont
             for (result, 0..) |class, i| switch (class) {
                 .memory => return Class.stack,
                 .x87up => if (i == 0 or result[i - 1] != .x87) return Class.stack,
-                else => continue,
+                else => {},
             };
             // "If the size of the aggregate exceeds two eightbytes and the first eight-
-            // byte isn’t SSE or any other eightbyte isn’t SSEUP, the whole argument
+            // byte isn't SSE or any other eightbyte isn't SSEUP, the whole argument
             // is passed in memory."
             if (ty_size > 16 and (result[0] != .sse or
                 std.mem.indexOfNone(Class, result[1..], &.{ .sseup, .none }) != null)) return Class.stack;
 
             // "If SSEUP is not preceded by SSE or SSEUP, it is converted to SSE."
-            for (&result, 0..) |*item, i| {
-                if (item.* == .sseup) switch (result[i - 1]) {
-                    .sse, .sseup => continue,
-                    else => item.* = .sse,
-                };
-            }
+            for (&result, 0..) |*class, i| switch (class.*) {
+                .sseup => switch (result[i - 1]) {
+                    .sse, .sseup => {},
+                    else => class.* = .sse,
+                },
+                .float => if (i + 1 < result.len) switch (result[i + 1]) {
+                    .none => {},
+                    else => class.* = .float_combine,
+                },
+                else => {},
+            };
             return result;
         },
         .array => {
diff --git a/test/c_abi/cfuncs.c b/test/c_abi/cfuncs.c
index bd925d58b9..fc708f343e 100644
--- a/test/c_abi/cfuncs.c
+++ b/test/c_abi/cfuncs.c
@@ -2948,7 +2948,6 @@ void run_c_tests(void) {
 #if !defined(__riscv)
 #if !defined(__s390x__)
 #if !defined(__i386__)
-#if !defined(__x86_64__)
     {
         struct Struct_f32a8_f32a8 s = zig_ret_struct_f32a8_f32a8();
         assert_or_panic(s.a == 1.25f);
@@ -2962,7 +2961,6 @@ void run_c_tests(void) {
 #endif
 #endif
 #endif
-#endif
 
 #if !(defined(__arm__) && defined(__SOFTFP__))
 #if !defined(__loongarch__) && !defined(__mips64__)
diff --git a/test/c_abi/main.zig b/test/c_abi/main.zig
index 353804e480..1f221260b4 100644
--- a/test/c_abi/main.zig
+++ b/test/c_abi/main.zig
@@ -554,22 +554,14 @@ const Struct_f32a8_f32a8 = extern struct {
     b: f32 align(8),
 };
 
-comptime {
-    skip: {
-        if (builtin.zig_backend == .stage2_x86_64) break :skip;
+export fn zig_ret_struct_f32a8_f32a8() Struct_f32a8_f32a8 {
+    return .{ .a = 1.25, .b = 2.75 };
+}
 
-        _ = struct {
-            export fn zig_ret_struct_f32a8_f32a8() Struct_f32a8_f32a8 {
-                return .{ .a = 1.25, .b = 2.75 };
-            }
-
-            export fn zig_struct_f32a8_f32a8(s: Struct_f32a8_f32a8, f: f32) void {
-                expect(s.a == 3.125) catch @panic("test failure");
-                expect(s.b == 4.375) catch @panic("test failure");
-                expect(f == 5.5) catch @panic("test failure");
-            }
-        };
-    }
+export fn zig_struct_f32a8_f32a8(s: Struct_f32a8_f32a8, f: f32) void {
+    expect(s.a == 3.125) catch @panic("test failure");
+    expect(s.b == 4.375) catch @panic("test failure");
+    expect(f == 5.5) catch @panic("test failure");
 }
 
 extern fn c_ret_struct_f32a8_f32a8() Struct_f32a8_f32a8;
@@ -584,7 +576,6 @@ test "C ABI struct f32 align(8), f32 align(8)" {
     if (builtin.cpu.arch.isRISCV()) return error.SkipZigTest;
     if (builtin.cpu.arch == .s390x) return error.SkipZigTest;
     if (builtin.cpu.arch == .x86) return error.SkipZigTest;
-    if (builtin.cpu.arch == .x86_64) return error.SkipZigTest;
 
     const s = c_ret_struct_f32a8_f32a8();
     try expect(s.a == 6.625);