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To:	cygwin AT cygwin DOT com
From:	Herb Schwetman <hds AT mesquite DOT com>
Subject:	gcc on 64-bit cywin - problems with __builtin_frame_address and __builtin_return_address
Message-ID:	<55E85B00.8020807@mesquite.com>
Date:	Thu, 3 Sep 2015 09:36:48 -0500
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Hello

The 64-bit gcc procedure prolog pushes registers onto the stack in a 
manner that is different from the 32-bit gcc version.

Furthermore, this appears to be unique to Cygwin64, and this does not 
happen on Linux.

This shows up when you use __builtin_frame_address(n) and 
__builtin_return_address(n).

This is illustrated with the attached C++  program.  If the local 
variables in the sub() procedure are designated as register variables, 
the sub procedure does not find the frame address and return address of 
the caller (main).  If these variables are not designated a register 
variables, the sub procedure does find and print the correct values.

If you look at the generated code (-S option), you can see that the 
64-bit versions pushes the saved registers onto the stack before the 
frame pointer is updated, while the 32-bit version pushes the saved 
registers onto the stack after the frame pointer is updated.

64-bit version

         .seh_proc       _Z3subv
_Z3subv:
.LFB7:
         pushq   %rbp
         .seh_pushreg    %rbp
         pushq   %rdi
         .seh_pushreg    %rdi
         pushq   %rsi
         .seh_pushreg    %rsi
         pushq   %rbx
         .seh_pushreg    %rbx
         movq    %rsp, %rbp
         .seh_setframe   %rbp, 0
         subq    $40, %rsp

32-bit version

         .def    __Z3subv;       .scl    2;      .type   32; .endef
__Z3subv:
LFB7:
         .cfi_startproc
         pushl   %ebp
         .cfi_def_cfa_offset 8
         .cfi_offset 5, -8
         movl    %esp, %ebp
         .cfi_def_cfa_register 5
         pushl   %edi
         pushl   %esi
         pushl   %ebx
         subl    $28, %esp

Herb Schwetman
Mesquite Software, Inc.




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