[Qemu-devel] [PATCH for-2.5 3/5] target-sh4: improve cmp/str instruction

[Aurelien Jarno <aurelien@aurel32.net>]

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.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
---
 target-sh4/translate.c | 17 +++++------------
 1 file changed, 5 insertions(+), 12 deletions(-)

diff --git a/target-sh4/translate.c b/target-sh4/translate.c
index a6f3f59..7d7d5e0 100644
--- a/target-sh4/translate.c
+++ b/target-sh4/translate.c
@@ -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);
 	}
-- 
2.1.4