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target-arm: Eliminate unnecessary zero-extend in disas_bitfield 

For !SF, this initial ext32u can't be optimized away by the
current TCG code generator.  (It would require backward bit
liveness propagation.)

But since the range of bits for !SF are already constrained by
unallocated_encoding, we'll never reference the high bits anyway.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-id: 1441909103-24666-10-git-send-email-rth@twiddle.net
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
This commit is contained in:
Richard Henderson 2015-09-14 14:39:48 +01:00 committed by Peter Maydell
parent 9924e85829
commit d3a77b42de
1 changed files with 5 additions and 1 deletions
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6
target-arm/translate-a64.c
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@ -3015,7 +3015,11 @@ static void disas_bitfield(DisasContext *s, uint32_t insn)
}
tcg_rd = cpu_reg(s, rd);
tcg_tmp = read_cpu_reg(s, rn, sf);
/* Suppress the zero-extend for !sf. Since RI and SI are constrained
to be smaller than bitsize, we'll never reference data outside the
low 32-bits anyway. */
tcg_tmp = read_cpu_reg(s, rn, 1);
/* Recognize the common aliases. */
if (opc == 0) { /* SBFM */