From b3b6d8ba0b33693f406a314d2dd10c68400cd8a4 Mon Sep 17 00:00:00 2001
From: Scott Graham <sgraham@gmail.com>
Date: Thu, 8 Jan 2026 12:32:59 -0800
Subject: winabi: fix allocation of parameters to regs with hidden arg

In the presence of the hidden arg for return-by-value, the
registers used for natural arguments were incorrect.

(This should be applied on the 'winabi' branch.)
---
 amd64/winabi.c |  3 ++-
 test/abi9.ssa  | 20 ++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)
 create mode 100644 test/abi9.ssa

diff --git a/amd64/winabi.c b/amd64/winabi.c
index 82829bc..a7d0a1a 100755
--- a/amd64/winabi.c
+++ b/amd64/winabi.c
@@ -630,6 +630,7 @@ static RegisterUsage lower_func_parameters(Fn* func) {
   // when adding to it.
   curi = &insb[NIns];
 
+  int reg_counter = 0;
   RegisterUsage reg_usage = {0};
   if (func->retty >= 0) {
     bool by_copy = type_is_by_copy(&typ[func->retty]);
@@ -639,6 +640,7 @@ static RegisterUsage lower_func_parameters(Fn* func) {
       Ref ret_ref = newtmp("abi.ret", Kl, func);
       emit(Ocopy, Kl, ret_ref, TMP(RCX), R);
       func->retr = ret_ref;
+      ++reg_counter;
     }
   }
   Ref env = R;
@@ -650,7 +652,6 @@ static RegisterUsage lower_func_parameters(Fn* func) {
   // Copy from the registers or stack slots into the named parameters. Depending
   // on how they're passed, they either need to be copied or loaded.
   ArgClass* arg = arg_classes;
-  int reg_counter = 0;
   uint slot_offset = SHADOW_SPACE_SIZE / 4 + 4;
   for (Ins* instr = start_of_params; instr < end_of_params; ++instr, ++arg) {
     switch (arg->style) {
diff --git a/test/abi9.ssa b/test/abi9.ssa
new file mode 100644
index 0000000..cb70028
--- /dev/null
+++ b/test/abi9.ssa
@@ -0,0 +1,20 @@
+type :obj = { l, l, l, l }
+
+export
+function :obj $f(l %self) {
+@_0
+        %_1 =l alloc8 16
+	storel 77, %_1
+        ret %_1
+}
+
+# >>> driver
+# #include <stdio.h>
+# typedef struct { long long a, b, c, d; } obj;
+# extern obj f();
+# int main() { obj ret = f(); printf("%lld\n", ret.a); return 0; }
+# <<<
+
+# >>> output
+# 77
+# <<<
-- 
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