[cfe-commits] r91073 - in /cfe/trunk: lib/Sema/SemaDecl.cpp test/Sema/return.c

[Chris Lattner]

On Dec 10, 2009, at 5:42 PM, Mike Stump wrote:

> On Dec 10, 2009, at 4:08 PM, Chris Lattner wrote:
>> On Dec 10, 2009, at 2:57 PM, Mike Stump wrote:
>> 
>>> Author: mrs
>>> Date: Thu Dec 10 16:57:48 2009
>>> New Revision: 91073
>>> 
>>> URL: http://llvm.org/viewvc/llvm-project?rev=91073&view=rev
>>> Log:
>>> Don't complain about falling off the end of a function with an asm
>>> block, if the function is supposed to return a value as we don't know
>>> exactly what the asm code does.
>> 
>> Why?
> 
> Because people write code like:
> 
>  void* t4(void) { __asm mov eax, fs:[0x10] }

That is not gnu asm syntax.  Also, you're not fixing the right thing here.  Several issues:

1. Setting eax is guaranteed to set the return value, the compiler could clobber it after the asm.
2. The bigger issue is that you're *changing the CFG* to avoid a *data* issue.

In either case, your code is not right.

>> A fix to silence the warning is to use __builtin_unreachable() after the asm.
> 
> But that point is reachable.

Then why are you changing the CFG?

> 
> Also, int foo() { asm("a: jump a"); } doesn't fall off the end.  Essentially, trying to understand what is inside the asm is a dangerous thing.

You're right, which is why we should follow gcc's policy of assuming it can fall through!

-Chris