target/riscv: Fix orc.b implementation

The earlier implementation fell into a corner case for bytes that were
0x01, giving a wrong result (but not affecting our application test
cases for strings, as an ASCII value 0x01 is rare in those...).

This changes the algorithm to:
 1. Mask out the high-bit of each bytes (so that each byte is <= 127).
 2. Add 127 to each byte (i.e. if the low 7 bits are not 0, this will overflow
    into the highest bit of each byte).
 3. Bitwise-or the original value back in (to cover those cases where the
    source byte was exactly 128) to saturate the high-bit.
 4. Shift-and-mask (implemented as a mask-and-shift) to extract the MSB of
    each byte into its LSB.
 5. Multiply with 0xff to fan out the LSB to all bits of each byte.

Fixes: d7a4fcb034 ("target/riscv: Add orc.b instruction for Zbb, removing gorc/gorci")

Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Reported-by: Vincent Palatin <vpalatin@rivosinc.com>
Tested-by: Vincent Palatin <vpalatin@rivosinc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20211013184125.2010897-1-philipp.tomsich@vrull.eu
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

1 files changed, 8 insertions, 5 deletions

diff --git a/target/riscv/insn_trans/trans_rvb.c.inc b/target/riscv/insn_trans/trans_rvb.c.inc
index 185c3e9..3095624 100644
--- a/target/riscv/insn_trans/trans_rvb.c.inc
+++ b/target/riscv/insn_trans/trans_rvb.c.inc
@@ -249,13 +249,16 @@ static bool trans_rev8_64(DisasContext *ctx, arg_rev8_64 *a)
 static void gen_orc_b(TCGv ret, TCGv source1)
 {
     TCGv  tmp = tcg_temp_new();
-    TCGv  ones = tcg_constant_tl(dup_const_tl(MO_8, 0x01));
+    TCGv  low7 = tcg_constant_tl(dup_const_tl(MO_8, 0x7f));
 
-    /* Set lsb in each byte if the byte was zero. */
-    tcg_gen_sub_tl(tmp, source1, ones);
-    tcg_gen_andc_tl(tmp, tmp, source1);
+    /* Set msb in each byte if the byte was non-zero. */
+    tcg_gen_and_tl(tmp, source1, low7);
+    tcg_gen_add_tl(tmp, tmp, low7);
+    tcg_gen_or_tl(tmp, tmp, source1);
+
+    /* Extract the msb to the lsb in each byte */
+    tcg_gen_andc_tl(tmp, tmp, low7);
     tcg_gen_shri_tl(tmp, tmp, 7);
-    tcg_gen_andc_tl(tmp, ones, tmp);
 
     /* Replicate the lsb of each byte across the byte. */
     tcg_gen_muli_tl(ret, tmp, 0xff);