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Subject: [Bug target/55171] incorrect virtual thunk on mingw
Date: Sun, 11 Nov 2012 14:30:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55171

Mikael Pettersson <mikpe at it dot uu.se> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mikpe at it dot uu.se

--- Comment #5 from Mikael Pettersson <mikpe at it dot uu.se> 2012-11-11 14:30:14 UTC ---
It's caused by "[patch i386]: Set for method-functions default
calling-convention to thiscall for 32-bit msabi" in r171890:
http://gcc.gnu.org/ml/gcc-patches/2011-04/msg00066.html
http://gcc.gnu.org/ml/gcc-cvs/2011-04/msg00082.html

With x86_64-w64-mingw32-g++ -m32 -O2 there are numerous code generation
differences in r171890 for this test case (as expected with the changed calling
convention), but looking only at the virtual thunk we see:

--- pr55171.s-r171889   2012-11-11 14:51:38.000000000 +0100
+++ pr55171.s-r171890   2012-11-11 15:00:58.000000000 +0100
...
 __ZTv0_n16_NK7Derived3fooEv:
-       movl    8(%esp), %eax
-       movl    (%eax), %ecx
-       addl    -16(%ecx), %eax
-       movl    %eax, 8(%esp)
+       movl    4(%esp), %eax
+       movl    (%eax), %eax
+       addl    -16(%eax), %eax
+       movl    %eax, 4(%esp)
        jmp     LTHUNK2
...

The two code sequences are clearly not equivalent (ignore the stack position of
the pointer being adjusted).

Running the test case confirms: with g++ @ r171889 it succeeds, but with g++ @
r171890 it gets a runtime failure.