------- Comment #9 from ubizjak at gmail dot com  2008-08-02 13:08 -------
Hm, IA-32 Intel® Architecture Software Developer’s Manual says about movq:

Operation
MOVQ instruction when operating on MMX technology registers and memory
locations:
   DEST ← SRC;
MOVQ instruction when source and destination operands are XMM registers:
   DEST[63:0] ← SRC[63:0];
MOVQ instruction when source operand is XMM register and destination
operand is memory location:
   DEST ← SRC[63:0];
MOVQ instruction when source operand is memory location and destination
operand is XMM register:
   DEST[63:0] ← SRC;
   DEST[127:64] ← 0000000000000000H;

Please note, that the documentation doesn't say anything about clearing
destination bits [127:64] when both source and destination are in XMM register.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36992