Re: [Qemu-devel] [PATCH 1/2] tcg-x86_64: Special-case all 32-bit AND operands.

[Aurelien Jarno <aurelien@aurel32.net>]

On Thu, Jan 14, 2010 at 08:05:58AM -0800, Richard Henderson wrote:
> On 01/14/2010 07:57 AM, Aurelien Jarno wrote:
> >On Tue, Jan 05, 2010 at 04:03:00PM -0800, Richard Henderson wrote:
> >>This avoids an unnecessary REX.W prefix when dealing with AND
> >>operands that fit into a 32-bit quantity.  The most common change
> >>actually seen is movz[wb]q ->  movz[wb]l.
> >>
> >>Similarly, avoid REXW in ext{8,16}u_i64 tcg opcodes.
> >>
> >>Signed-off-by: Richard Henderson<rth@twiddle.net>
> >>---
> >>  tcg/x86_64/tcg-target.c |   26 ++++++++------------------
> >>  1 files changed, 8 insertions(+), 18 deletions(-)
> >>
> >>diff --git a/tcg/x86_64/tcg-target.c b/tcg/x86_64/tcg-target.c
> >>index 2339091..f584c94 100644
> >>--- a/tcg/x86_64/tcg-target.c
> >>+++ b/tcg/x86_64/tcg-target.c
> >>@@ -426,24 +426,18 @@ static inline void tgen_arithi64(TCGContext *s, int c, int r0, int64_t val)
> >>      } else if ((c == ARITH_ADD&&  val == -1) || (c == ARITH_SUB&&  val == 1)) {
> >>          /* dec */
> >>          tcg_out_modrm(s, 0xff | P_REXW, 1, r0);
> >>-    } else if (val == (int8_t)val) {
> >>-        tcg_out_modrm(s, 0x83 | P_REXW, c, r0);
> >>-        tcg_out8(s, val);
> >>-    } else if (c == ARITH_AND&&  val == 0xffu) {
> >>-        /* movzbl */
> >>-        tcg_out_modrm(s, 0xb6 | P_EXT | P_REXW, r0, r0);
> >>-    } else if (c == ARITH_AND&&  val == 0xffffu) {
> >>-        /* movzwl */
> >>-        tcg_out_modrm(s, 0xb7 | P_EXT | P_REXW, r0, r0);
> >>      } else if (c == ARITH_AND&&  val == 0xffffffffu) {
> >>          /* 32-bit mov zero extends */
> >>          tcg_out_modrm(s, 0x8b, r0, r0);
> >>+    } else if (c == ARITH_AND&&  (uint64_t)val<= 0xffffffffu) {
> >>+        /* AND with no high bits set can use a 32-bit operation.  */
> >>+        tgen_arithi32(s, c, r0, val);
> >
> >Do we really want to call tgen_arithi32() here, that will redo part of
> >the above tests again? It might be better to simply remove the REX.W
> >prefix above instead.
> 
> Pardon?  Do you mean the inc/dec tests?  Otherwise I don't see what
> "above tests again" you're talking about.
> 
> I am looking to handle more than 0xff, 0xffff with the new test in
> gen_arithi64 -- 0x1234 is an appropriate mask to shorten to 32-bit
> AND as well.  I have no idea if that answers your question.
> 

Yes it does, I missed that fact. Viewing your patch on this side gives
it more sense. I have applied it.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net