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target-sh4: improve cmp/str instruction

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Instead of testing bytes one by one, we can use the following trick
from https://graphics.stanford.edu/~seander/bithacks.html:

  haszero(v) = (v - 0x01010101) & ~v & 0x80808080

The subexpression v - 0x01010101, evaluates to a high bit set in any
byte whenever the corresponding byte in v is zero or greater than 0x80.
The sub-expression ~v & 0x80808080 evaluates to high bits set in bytes
where the byte of v doesn't have its high bit set (so the byte was less
than 0x80). Finally, by ANDing these two sub-expressions the result is
the high bits set where the bytes in v were zero, since the high bits
set due to a value greater than 0x80 in the first sub-expression are
masked off by the second.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
This commit is contained in:
Aurelien Jarno 2015-07-05 18:50:09 +02:00
parent 218fd7301f
commit eb6ca2b4a6
1 changed files with 5 additions and 12 deletions
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17
target-sh4/translate.c
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@ -688,18 +688,11 @@ static void _decode_opc(DisasContext * ctx)
{
TCGv cmp1 = tcg_temp_new();
TCGv cmp2 = tcg_temp_new();
tcg_gen_xor_i32(cmp1, REG(B7_4), REG(B11_8));
tcg_gen_andi_i32(cmp2, cmp1, 0xff000000);
tcg_gen_setcondi_i32(TCG_COND_EQ, cpu_sr_t, cmp2, 0);
tcg_gen_andi_i32(cmp2, cmp1, 0x00ff0000);
tcg_gen_setcondi_i32(TCG_COND_EQ, cmp2, cmp2, 0);
tcg_gen_or_i32(cpu_sr_t, cpu_sr_t, cmp2);
tcg_gen_andi_i32(cmp2, cmp1, 0x0000ff00);
tcg_gen_setcondi_i32(TCG_COND_EQ, cmp2, cmp2, 0);
tcg_gen_or_i32(cpu_sr_t, cpu_sr_t, cmp2);
tcg_gen_andi_i32(cmp2, cmp1, 0x000000ff);
tcg_gen_setcondi_i32(TCG_COND_EQ, cmp2, cmp2, 0);
tcg_gen_or_i32(cpu_sr_t, cpu_sr_t, cmp2);
tcg_gen_xor_i32(cmp2, REG(B7_4), REG(B11_8));
tcg_gen_subi_i32(cmp1, cmp2, 0x01010101);
tcg_gen_andc_i32(cmp1, cmp1, cmp2);
tcg_gen_andi_i32(cmp1, cmp1, 0x80808080);
tcg_gen_setcondi_i32(TCG_COND_NE, cpu_sr_t, cmp1, 0);
tcg_temp_free(cmp2);
tcg_temp_free(cmp1);
}