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From: Richard Henderson <rth@twiddle.net>
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Date: Thu, 7 Apr 2016 20:40:26 -0700
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Subject: Re: [Qemu-devel] [PATCH 01/11] tci: Fix build regression
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To: Stefan Weil <sw@weilnetz.de>, Sergey Fedorov <sergey.fedorov@linaro.org>, qemu-devel@nongnu.org
Cc: =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, Sergey Fedorov <serge.fdrv@gmail.com>, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Michael Roth <mdroth@linux.vnet.ibm.com>, Peter Maydell <peter.maydell@linaro.org>

On 04/07/2016 01:37 PM, Stefan Weil wrote:
> I just tested a variant with tcg_debug_assert. It creates less efficient
> code when debugging is disabled. Here is the code size for x86_64:
>
> with normal tcg_debug_assert:
>
>     text       data        bss        dec        hex    filename
>     8293          0        128       8421       20e5
> bin/ndebug/x86_64-linux-gnu,tci/x86_64-softmmu/tci.o
>
> In gdb I can also see assembler code for op_size at the beginning
> of the for loop in tcg_qemu_tb_exec. This slows down the interpreter.

Please file a gcc bug.

A branch across __builtin_unreachable is supposed to fold to a simple 
compile-time assert, providing extra information to the compiler but not 
generating code.


r~