From a37f8cfc805025019c93ae3114295eff363168a0 Mon Sep 17 00:00:00 2001
From: seyko <seyko2@gmail.com>
Date: Sun, 15 May 2016 21:10:06 +0300
Subject: [PATCH] short_call_convention patch from tcc bugzilla

    BUGZILLA:
    interfacing with other compilers

    extend the return value to the whole register if necessary.
    visual studio and gcc do not always set the whole eax register
    when assigning the return value of a function.

    We've encountered wrong execution results on i386 platforms with an
    application that uses both code compiled with TCC and code compiled
    with other compilers (namely: Visual Studio on Windows, and GCC on
    Linux).

    When calling a function that returns an integer value shorter than 32
    bits, TCC reads the return value from the whole EAX register,
    although the code generated by the other compilers can only sets AL
    for 8 bit values or AX for 16 bits values, and the rest of EAX can be
    anything.

    We worked around this with the attached patch on i386 for the version
    0.9.26, but we did not look at other platforms to find if there are
    similar issues.
---
 i386-gen.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/i386-gen.c b/i386-gen.c
index 77536f8..eace2df 100644
--- a/i386-gen.c
+++ b/i386-gen.c
@@ -354,6 +354,7 @@ static void gcall_or_jmp(int is_jmp)
 {
     int r;
     if ((vtop->r & (VT_VALMASK | VT_LVAL)) == VT_CONST) {
+        int rt;
         /* constant case */
         if (vtop->r & VT_SYM) {
             /* relocation case */
@@ -365,6 +366,30 @@ static void gcall_or_jmp(int is_jmp)
                           ind + 1, R_386_PC32, 0);
         }
         oad(0xe8 + is_jmp, vtop->c.i - 4); /* call/jmp im */
+        /* extend the return value to the whole register if necessary
+           visual studio and gcc do not always set the whole eax register
+           when assigning the return value of a function  */
+        rt = vtop->type.ref->type.t;
+        switch (rt & VT_BTYPE) {
+            case VT_BYTE:
+                if (rt & VT_UNSIGNED) {
+                    o(0xc0b60f); /* movzx %al, %eax */
+                }
+                else {
+                    o(0xc0be0f); /* movsx %al, %eax */
+                }
+                break;
+            case VT_SHORT:
+                if (rt & VT_UNSIGNED) {
+                    o(0xc0b70f); /* movzx %ax, %eax */
+                }
+                else {
+                    o(0xc0bf0f); /* movsx %ax, %eax */
+                }
+                break;
+            default:
+                break;
+        }
     } else {
         /* otherwise, indirect call */
         r = gv(RC_INT);