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A later x87 instruction such as fucomi/fucomip can then read a stale finite value where real hardware guarantees a NaN encoding, changing comparison results and flags. Set the high 16 bits to 0xffff when writing back to a MMX register (not memory). Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/4105 Signed-off-by: Simon Scherer Reviewed-by: Richard Henderson --- target/i386/tcg/emit.c.inc | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/target/i386/tcg/emit.c.inc b/target/i386/tcg/emit.c.inc index 473f415766..e2f0b4b1f7 100644 --- a/target/i386/tcg/emit.c.inc +++ b/target/i386/tcg/emit.c.inc @@ -352,6 +352,10 @@ static void gen_writeback(DisasContext *s, X86DecodedI= nsn *decode, int opn, TCGv } break; case X86_OP_MMX: + if (!op->has_ea) { + tcg_gen_st16_i32(tcg_constant_i32(0xffff), tcg_env, + MMX_OFFSET(op->n) + offsetof(floatx80, high)); + } break; case X86_OP_SSE: if (!op->has_ea && (s->prefix & PREFIX_VEX) && op->ot <=3D MO_128)= { --=20 2.53.0