From 36e929101bc88f161f57c4e67974287c3a4f79a4 Mon Sep 17 00:00:00 2001
From: agra <alex@swipelab.co>
Date: Tue, 19 May 2026 11:22:56 +0300
Subject: [PATCH] issue-0036: 16-byte integer-only struct by value trips LLVM
 verifier

Surfaced while writing the ffi-02-small-struct.sx baseline. The sx
#foreign decl lowers `{ s64, s64 }` (and other 16-byte integer-only
shapes like `{ s32, s32, s32, s32 }`) to `[2 x i64]` for the small-
struct register-pair ABI on AAPCS64 / SysV AMD64, but the call site
loads the struct as `{ i64, i64 }`. The two types must agree for the
LLVM verifier to accept the call:

  Call parameter type does not match function signature!
    %load = load { i64, i64 }, ptr %alloca, align 8
  [2 x i64]  %call = call [2 x i64] @issue0036_swap({ i64, i64 } %load)

Float-only 16-byte aggregates (e.g. Vec4f) work because they route
through the HFA path which keeps the struct representation. See
examples/ffi-02-small-struct.sx for the working cases.

Phase 1's #foreign lowering rework is the natural place to unify
these representations; check there before fixing inline.
---
 examples/issue-0036.sx          | 38 +++++++++++++++++++++++++++++++++
 tests/expected/issue-0036.exit  |  1 +
 tests/expected/issue-0036.txt   |  3 +++
 vendors/issue_0036/issue_0036.c |  9 ++++++++
 4 files changed, 51 insertions(+)
 create mode 100644 examples/issue-0036.sx
 create mode 100644 tests/expected/issue-0036.exit
 create mode 100644 tests/expected/issue-0036.txt
 create mode 100644 vendors/issue_0036/issue_0036.c

diff --git a/examples/issue-0036.sx b/examples/issue-0036.sx
new file mode 100644
index 0000000..ff73159
--- /dev/null
+++ b/examples/issue-0036.sx
@@ -0,0 +1,38 @@
+// 16-byte integer-only struct by value through `#foreign` trips
+// the LLVM verifier:
+//
+//   Call parameter type does not match function signature!
+//     %loadN = load { i64, i64 }, ptr %allocaM, align 8
+//   [2 x i64]  %callN = call [2 x i64] @issue0036_swap({ i64, i64 } %load...)
+//
+// The foreign-decl side lowers `Pair64` to `[2 x i64]` (ABI-coercion
+// for the small-struct register-pair return on AAPCS64 / SysV AMD64),
+// while the call site loads the sx struct as `{ i64, i64 }`. The two
+// representations need to agree.
+//
+// Float-only aggregates of the same total size (e.g. Vec4f, 4×f32 =
+// 16 B) work today because they're routed through the HFA path which
+// keeps the struct representation. See `examples/ffi-02-small-struct.sx`
+// for the working cases.
+//
+// Expected once fixed: print "ok = true" with exit 0. Today this
+// example fails at codegen with a non-zero exit; the snapshot captures
+// that failure so the test suite stays honest.
+
+#import "modules/std.sx";
+
+#import c {
+    #source "vendors/issue_0036/issue_0036.c";
+};
+
+Pair64 :: struct { a: s64; b: s64; }
+
+issue0036_swap :: (p: Pair64) -> Pair64 #foreign;
+
+main :: () -> s32 {
+    p : Pair64 = .{ a = 1, b = 2 };
+    q := issue0036_swap(p);
+    print("swap = ({}, {})\n", q.a, q.b);
+    print("ok = {}\n", q.a == 2 and q.b == 1);
+    0;
+}
diff --git a/tests/expected/issue-0036.exit b/tests/expected/issue-0036.exit
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/tests/expected/issue-0036.exit
@@ -0,0 +1 @@
+1
diff --git a/tests/expected/issue-0036.txt b/tests/expected/issue-0036.txt
new file mode 100644
index 0000000..87e147d
--- /dev/null
+++ b/tests/expected/issue-0036.txt
@@ -0,0 +1,3 @@
+LLVM verification failed: Call parameter type does not match function signature!
+  %load = load { i64, i64 }, ptr %alloca5, align 8
+ [2 x i64]  %call = call [2 x i64] @issue0036_swap({ i64, i64 } %load)
diff --git a/vendors/issue_0036/issue_0036.c b/vendors/issue_0036/issue_0036.c
new file mode 100644
index 0000000..3013dcd
--- /dev/null
+++ b/vendors/issue_0036/issue_0036.c
@@ -0,0 +1,9 @@
+// Repro for issue-0036: 16-byte integer-only struct by-value through
+// #foreign. See examples/issue-0036.sx for details.
+
+typedef struct { long long a; long long b; } Pair64;
+
+Pair64 issue0036_swap(Pair64 p) {
+    Pair64 r = { p.b, p.a };
+    return r;
+}