Strange prolog function issue

Hi all, 
 
 
I have here a very strange issue.
 
We have a function using a local std::string, exceptions, and a alloca.
 
When the exception is thrown, the unwind mecanism tries to delete the std::string, but it deletes it at a wrong address( there is a 0x590 offset ).
 
We debugged this problem for two days, and it only happens when the alloca is present.
In this case, the prolog of the function is of this type :
 
00F60F00  push        ebp  
00F60F01  lea         ebp,[esp-590h]                 ***** NOTICE THE STRANGE EBP VALUE HERE
00F60F08  push        0FFFFFFFFh 
00F60F0A  push        offset __ehhandler$?CompileOneVariableDeclaration@AIE_cl_Compiler@@AAEXXZ (139B888h) 
00F60F0F  mov         eax,dword ptr fs:[00000000h] 
00F60F15  push        eax  
00F60F16  mov         dword ptr fs:[0],esp 
00F60F1D  sub         esp,5ECh 
00F60F23  mov         eax,dword ptr [___security_cookie (16A1EECh)] 
00F60F28  push        ebx  
00F60F29  push        esi  
00F60F2A  mov         dword ptr [ebp+580h],eax 
00F60F30  push        edi  
00F60F31  mov         ebx,ecx 
 
 
As without the alloca, the prolog is like :
00402580  push        ebp  
00402581  mov         ebp,esp            *********** HERE, AS ALWAYS, EBP IS SET TO ESP
00402583  push        0FFFFFFFFh 
00402585  push        offset __ehhandler$?MyFunc@@YAXXZ (420738h) 
0040258A  mov         eax,dword ptr fs:[00000000h] 
00402590  push        eax  
00402591  mov         dword ptr fs:[0],esp 
00402598  sub         esp,34h 
0040259B  mov         eax,dword ptr [___security_cookie (429608h)] 
004025A0  push        ebx  
004025A1  push        esi  
004025A2  mov         dword ptr [ebp-10h],eax 
004025A5  push        edi  
 
Note that in our crashing case, ebp is set to esp - 590 ( the offset value ), and that in the other case, it is just set to esp.
 
 
I could not reproduce this ebp offset that makes the exception handler fail in a small sample. 
So do you know in which case the compiler does not put esp in ebp, but put an offset in it ?
And do you know how to fix it ( other than what we're doing : stopping using alloca with exceptions ), if it is a kind a known issue ?
 
 
Thanks for any help, 
 
Emmanuel

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