[LLVMdev] built-in longjmp and setjmp

[Jim Grosbach]

The builtins are for internal compiler use in the context of SjLj exception handling. Any other use, including any direct calls of the builtins in user code, are a bad idea with no guaranteed behaviour. That they're exposed at all is, again, for historical purposes. Don't use them. 

-Jim

On Apr 27, 2011, at 3:45 PM, Akira Hatanaka wrote:

> Okay. I understand builtin functions do not have to behave exactly the same way as standard library functions. What I wanted to know is what should the code generated by llvm (clang + llc) look like (I am working on the Mips back-end now). I guess there should be a behavior users expect to see who are using __builtin_setjmp/longjmp even they aren't the same as library functions. If the code generated by arm-darwin is acceptable, does it mean that you can freely move code that accesses a global variable above the call to setjmp?
> 
> On Wed, Apr 27, 2011 at 2:03 PM, Jim Grosbach <grosbach at apple.com> wrote:
> There is no C standard to follow for these builtins. You are expecting them to behave as if they were the standard library calls. They are not equivalent and the naming similarity is an unfortunate historical artifact. Use the standard library functions instead.
> 
> -Jim
> 
> On Apr 27, 2011, at 1:22 PM, Akira Hatanaka wrote:
> 
> > I declared gi2 as "volatile" and I think gi2 is still incremented once.
> > Here is a snippet of the code. Line 39 - 42 increments gi2.
> >
> > According to the standard, shouldn't ++gi2 be executed twice regardless of whether gi2 is volatile or not? Isn't the missing chain from EH_SJLJ_SETJMP node to load/store nodes that access gi2 causing this problem (please see attached file in my previous email)?
> >
> > # line 39 - 47
> >  ldr r1, LCPI1_1
> >  ldr r2, [r1]
> >  add r2, r2, #1
> >  str r2, [r1]
> >  add r4, pc, #8              @ eh_setjmp begin
> >  str r4, [r0, #4]
> >  mov r0, #0
> >  add pc, pc, #0
> >  mov r0, #1                  @ eh_setjmp end
> >
> >  ...
> > LCPI1_1:
> >   .long _gi2
> >   .align  2
> >
> >
> > On Wed, Apr 27, 2011 at 11:38 AM, Akira Hatanaka <ahatanak at gmail.com> wrote:
> > I have another basic question about setjmp/longjmp.
> >
> > When I compile and run the following program, is it expected that global variable gi2 will be incremented twice? It seems that the code generated with clang and llc increments it only once (line 37-43 of attached file).
> >
> > $ clang setjmp6.c -o setjmp6.arm.ll -emit-llvm -O3 -S -ccc-host-triple arm-unknown-darwin -ccc-clang-archs arm
> > $ llc setjmp6.arm.ll -o setjmp6.arm.s
> >
> >
> > #include <stdio.h>
> > #include <stdlib.h>
> > void *buf[20];
> >
> > int gi2 = 0;
> >
> >
> > void __attribute__ ((noinline)) sub2 (void)
> > {
> >   __builtin_longjmp (buf, 1);
> >
> > }
> >
> > int
> > main (int argc, char **argv)
> > {
> >   int n = atoi (argv[1]);
> >   int r = __builtin_setjmp (buf);
> >   ++gi2;
> >
> >   if (r)
> >     {
> >       printf ("setjmp %d\n", n + gi2);
> >       return 0;
> >     }
> >
> >   sub2 ();
> >
> >   return 0;
> > }
> >
> >
> > On Wed, Apr 13, 2011 at 10:05 AM, Jakob Stoklund Olesen <stoklund at 2pi.dk> wrote:
> >
> > On Apr 13, 2011, at 9:51 AM, Akira Hatanaka wrote:
> >
> > > int
> > > main (int argc, char** argv)
> > > {
> > >   int n = atoi(argv[1]), r;
> > >
> > >   if ((r = setjmp (buf)))
> > >     {
> > >       printf("n = %d\n", n);
> > >       return 0;
> > >     }
> >
> >
> > /jakob
> >
> >
> >
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>