[llvm-commits] [llvm] r106662 - /llvm/trunk/lib/Target/ARM/ARMISelLowering.cpp

[Dale Johannesen]

On Jun 23, 2010, at 11:51 PMPDT, Chris Lattner wrote:
> On Jun 23, 2010, at 11:52 AM, Dale Johannesen wrote:
>> Author: johannes
>> Date: Wed Jun 23 13:52:34 2010
>> New Revision: 106662
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=106662&view=rev
>> Log:
>> Do not do tail calls to external symbols.  If the
>> branch turns out to be ARM-to-Thumb or vice versa
>> the linker cannot resolve this.  8120438.
>>
>> If this optimization is going to be useful we probably
>> need a compiler flag "assume callees are same architecture"
>> or something like that.
>
> Wow, this is a pretty bad limitation.  Does GCC have the same  
> limitation?

The limitation is imposed by the linker, not the compiler.  Yes,  
gcc-4.2 on darwin behaves the same way:  it does tail calls to  
functions defined in the same module, but not external calls.

As Bob says, doing tail call on ARM-to-thumb calls is a loser, as can  
be seen from Rafael's example, which executes an extra instruction  
over doing the call normally.   However I would guess most external  
calls will not be cross-architecture in practice, so it might win  
pragmatically (apparently the GNU people think so, I wonder if they  
have any actual data behind this belief?).