Some assembly fails in clang and succeeds in gcc

Clang Frontend
#1

I am looking at one project that doesn't compile with clang++.

Source in __asm__ __volatile__(...) after preprocessor looks like this:
<...skipped...>"mov [r8+((1024+7-(15))-((1024+7-(15))/(8))*(8))*4]" ", " "ecx" ";"

  "\n" "1" ":"
  "add rsi" ", " "4*16" ";"
<...skipped...>

This place breaks with this message:
sha.cpp:340:2: error: invalid instruction mnemonic 'prefix'
         ASL(1)
         ^
./cpu.h:224:17: note: expanded from macro 'ASL'
         #define ASL(x) GNU_ASL(x)
                        ^
./cpu.h:218:26: note: expanded from macro 'GNU_ASL'
         #define GNU_ASL(x) "\n" #x ":"
                                 ^
<scratch space>:24:2: note: expanded from here
"1"
  ^

However, there is the previous instance of the similar assembly that clang doesn't complain about:
  "movdqa xmm1" ", " "[rcx+1*16]" ";"
  "\n" "0" ":"
  "movdqa [r8+((1024+7-(0+3))-((1024+7-(0+3))/(8))*(8))*4]" ", " "xmm1" ";"

gcc compiles both places.

What is wrong with the first place?

Yuri

#2

So, 1: is supposed to be a label? I suspect that is not going to work. Consider:

$ cat t.cpp
int main() {
asm (“1: jmp 1”);
}

$ clang -cc1 t.cpp -S -o - | grep -A10 main:

_main:
#APP
Ltmp0:
jmp 1 # Not right…
#NO_APP
movl $0, %eax
retl

$ clang -cc1 t.cpp -emit-obj -o t.o
Assertion failed: Target.getSymA() && “Relocation must reference a symbol!”, file …\lib\MC\WinCOFFObjectWriter.cpp, line 714

So, clearly we have bugs. As a workaround, try using textual labels that start with ‘L’ so they don’t appear in the symbol table.

#3

However, in my case the corresponding jump label is 1b, and generated assembly is right:
$ cat t.cpp
int main() {
   asm ("1: jmp 1b");
}

$ clang -cc1 t.cpp -S -o - | grep -A10 main:
main:
         #APP
.Ltmp0:
         jmp .Ltmp0
         #NO_APP
         movl $0, %eax
         retq
.Ltmp1:
         .size main, .Ltmp1-main

I limited the testcase to this small form.
$ cat testcase.c
void func(void *state) {
__asm__ __volatile__
(
         "\n" "1" ":" "jb 1b;"
         ".att_syntax prefix;"
         :
         : "c" (state)
         : "memory", "cc", "%eax"
);
}

void use() {
   func(0);
}

$ clang testcase.c
testcase.c:5:15: error: invalid instruction mnemonic 'prefix'
         "\n" "1" ":" "jb 1b;"
               ^
<inline asm>:2:21: note: instantiated into assembly here
1:jb 1b;.att_syntax prefix;
                     ^~~~~~~
1 error generated.

gcc-4.8.0 compiles it, and clang rev.214386 doesn't.

Yuri

#4

This is http://llvm.org/bugs/show_bug.cgi?id=18916

#5

Should be fixed in r215031.