:[diStorm}: REP+REX prefix ordering

[Sanjay Patel]

I just found a bit of a problem decoding a multi-prefix SSE instruction from
GCC. It produced this byte sequence:

0x55, 
0x48, 0x89, 0xe5,
0x48, 0x89, 0x7d, 0xf8,
0xf2, 0x48, 0x0f, 0x2a, 0x4d, 0xf8,

Which should be:
push rbbp
mov rbp, rsp
cvtsi2sd xmm1, -8(rbp)

DiStorm got confused by the 0xf2, 0x48 prefix sequence, so it decoded the
last instruction as a 'cvtpi2ps'.

Note that the REX prefix and the SSE prefix are swapped from the normal way
you would expect...but it appears this is legal codegen.

--Sanjay




 
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