bugTiny C Compiler - Bugs: bug #48900, Wrong code generation on Windows...


bug #48900: Wrong code generation on Windows for external variables

Submitted by:  Shixin Zeng <shixinzeng>
Submitted on:  Thu 25 Aug 2016 04:18:19 PM UTC  
Category:  None Severity:  3 - Normal
Item Group:  None Status:  None
Privacy:  Public Assigned to:  None
Open/Closed:  Open

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Thu 25 Aug 2016 06:29:53 PM UTC, comment #2:

Somebody please close it, I can't find anywhere to change the status.

Shixin Zeng <shixinzeng>
Thu 25 Aug 2016 06:28:13 PM UTC, comment #1:

My bad, the extern variable needs _attribute_((dllimport)) to make it work.

After change the code to:

extern _attribute_((dllimport)) long long global;

it works fine.

Shixin Zeng <shixinzeng>
Thu 25 Aug 2016 04:18:19 PM UTC, original submission:

I found this while testing this program:
#include <stdlib.h>
#include "libtcc.h"

int global = 100;
int main()
int m_size;
void *m;
void (*p) (void);
TCCState *s = tcc_new();
tcc_set_output_type(s, TCC_OUTPUT_MEMORY);
tcc_set_options(s, "-nostdinc -nostdlib");
tcc_compile_string(s, "extern long long global;\nvoid printf(const char*fmt, ...);\nvoid p() {__asm__ volatile (\"int $0x03\");\nprintf(\"global: %d\n\", global);}\n");
#ifdef WIN32
tcc_add_library_path(s, "c:\\Users\\user\\tcc");
tcc_add_library(s, "tcc1");
tcc_add_library(s, "ucrtbase");
tcc_add_library(s, "msvcrt");
tcc_add_library_path(s, "/home/zsx/work/r3-build/tcc");
tcc_add_library(s, "tcc1");
tcc_add_symbol(s, "global", &global);
m_size = tcc_relocate(s, NULL);
m = malloc(m_size);
tcc_relocate(s, m);
p = tcc_get_symbol(s, "p");
p ();

return 0;


What I tried to do was passing the global variable to the tcc compiled code, and print it out.
On Linux, it worked fine (outputs 100), on Windows it's not(outputs some random number, like -866870476).

I looked at the assembly code generated on both platforms, and I found this difference:

0x0000000000668700: push rbp
0x0000000000668701: mov rbp,rsp
0x0000000000668704: sub rsp,0x0
0x000000000066870b: int3
=> 0x000000000066870c: mov rax,QWORD PTR [rip+0x45] # 0x668758
0x0000000000668713: mov rax,QWORD PTR [rax] #reading memory pointed by rax. The content of that memory location is address of "global" passed by tcc_add_symbol.
0x0000000000668716: mov rsi,rax
0x0000000000668719: lea rax,[rip+0x10] # 0x668730
0x0000000000668720: mov rdi,rax
0x0000000000668723: mov eax,0x0
0x0000000000668728: call 0x668780
0x000000000066872d: leave
0x000000000066872e: ret

000002697FA51A50 push rbp
000002697FA51A51 mov rbp,rsp
000002697FA51A54 sub rsp,20h
000002697FA51A5B int 3
000002697FA51A5C mov rax,qword ptr [2697FA51ADCh]
000002697FA51A63 mov r11,rax #Copy rax to r11, should be "move r11, QWORD PTR [rax]", I think.
000002697FA51A66 lea rax,[2697FA51A90h]
000002697FA51A6D mov r10,rax
000002697FA51A70 mov rcx,r10
000002697FA51A73 mov rdx,r11
000002697FA51A76 call 000002697FA51A88
000002697FA51A7B leave
000002697FA51A7C ret

Shixin Zeng <shixinzeng>


(Note: upload size limit is set to 16384 kB, after insertion of the required escape characters.)

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