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From: | Fabrice Bellard |
Subject: | Re: [Qemu-devel] Emulation differences, qemu-system-x86_64 vs Athlon64 |
Date: | Thu, 13 Apr 2006 00:18:05 +0200 |
User-agent: | Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.3) Gecko/20040913 |
Julian Seward wrote:
Recently I've been playing with CVS qemu-system (softmmu) on amd64 and had some stability problems. I decided to run Valgrind's amd64 instruction-set tests (derived from qemu's) to see if they picked up anything. Resulting diffs are attached. There are a bunch of differences for the C flag for rotates (rol/ror) by multiples of the word size. I don't think these are significant, but who knows.
This is a bug in QEMU. From the manuals, the C flag must be updated even if the resulting shift is zero. I just modified the QEMU tests to catch this problem (it is not x86_64 specific).
Perhaps more worryingly are the 20 or so lines at the bottom of the diff. These I believe are for double-to-int/short conversions for a value which is out of range for an int/short; the hardware produces 0x80000000/0x8000 respectively, which is the "integer indefinite"; QEMU produces zero. I can imagine some obscure routine somewhere checking for integer indefinite after conversion and being confused as a result.
I guess the problem comes from the usage of lrintl() on x86_64 in fpu/softfloat-native.c, but I cannot test it yet.
Fabrice.
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