[Qemu-devel] [PATCH] tcg/mips: fix add2

[Qemu-devel] [PATCH] tcg/mips: fix add2

[Aurelien Jarno]

The add2 code in the tcg_out_addsub2 function doesn't take into account
the case where rl == al == bl. In that case we can't compute the carry
after the addition. As it corresponds to a multiplication by 2, the
carry bit is the bit 31.

While this is a corner case, this prevents x86-64 guests to boot on a
MIPS host.

Cc: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
---
 tcg/mips/tcg-target.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tcg/mips/tcg-target.c b/tcg/mips/tcg-target.c
index 064db46..e97980d 100644
--- a/tcg/mips/tcg-target.c
+++ b/tcg/mips/tcg-target.c
@@ -1271,6 +1271,9 @@ static void tcg_out_addsub2(TCGContext *s, TCGReg rl, TCGReg rh, TCGReg al,
         if (cbl) {
             tcg_out_opc_imm(s, OPC_ADDIU, rl, al, bl);
             tcg_out_opc_imm(s, OPC_SLTIU, TCG_TMP0, rl, bl);
+        } else if (rl == al && rl == bl) {
+            tcg_out_opc_sa(s, OPC_SRL, TCG_TMP0, al, 31);
+            tcg_out_opc_reg(s, OPC_ADDU, rl, al, bl);
         } else {
             tcg_out_opc_reg(s, OPC_ADDU, rl, al, bl);
             tcg_out_opc_reg(s, OPC_SLTU, TCG_TMP0, rl, (rl == bl ? al : bl));
-- 
2.1.4

Re: [Qemu-devel] [PATCH] tcg/mips: fix add2

[Richard Henderson]

On 07/31/2015 08:31 AM, Aurelien Jarno wrote:
> The add2 code in the tcg_out_addsub2 function doesn't take into account
> the case where rl == al == bl. In that case we can't compute the carry
> after the addition. As it corresponds to a multiplication by 2, the
> carry bit is the bit 31.
>
> While this is a corner case, this prevents x86-64 guests to boot on a
> MIPS host.
>
> Cc: Richard Henderson<rth@twiddle.net>
> Signed-off-by: Aurelien Jarno<aurelien@aurel32.net>
> ---
>   tcg/mips/tcg-target.c | 3 +++
>   1 file changed, 3 insertions(+)

Reviewed-by: Richard Henderson <rth@twiddle.net>


r~