[Qemu-devel] [PATCH v2] tcg: fix dead computation for repeated input arguments

[Qemu-devel] [PATCH v2] tcg: fix dead computation for repeated input arguments

[Aurelien Jarno]

When the same temp is used twice or more as an input argument to a TCG
instruction, the dead computation code doesn't recognize the second use
as a dead temp. This is because the temp is marked as live in the same
loop where dead inputs are checked.

The fix is to split the loop in two parts. This avoid emitting a move
and using a register for the movcond instruction when used as "move if
true" on x86-64. This might bring more improvements on RISC TCG targets
which don't have outputs aliased to inputs.

Cc: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
---
 tcg/tcg.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

v1-v2
  1. Apply the same fix for helpers
  2. Add comments as suggested by Richard

diff --git a/tcg/tcg.c b/tcg/tcg.c
index 8b43bbb..3960eba 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -1378,16 +1378,20 @@ static void tcg_liveness_analysis(TCGContext *s)
                         memset(dead_temps, 1, s->nb_globals);
                     }
 
-                    /* input args are live */
+                    /* record arguments that die in this helper */
                     for (i = nb_oargs; i < nb_iargs + nb_oargs; i++) {
                         arg = args[i];
                         if (arg != TCG_CALL_DUMMY_ARG) {
                             if (dead_temps[arg]) {
                                 dead_args |= (1 << i);
                             }
-                            dead_temps[arg] = 0;
                         }
                     }
+                    /* input arguments are live for preceeding opcodes */
+                    for (i = nb_oargs; i < nb_oargs + nb_iargs; i++) {
+                        arg = args[i];
+                        dead_temps[arg] = 0;
+                    }
                     s->op_dead_args[oi] = dead_args;
                     s->op_sync_args[oi] = sync_args;
                 }
@@ -1522,12 +1526,16 @@ static void tcg_liveness_analysis(TCGContext *s)
                     memset(mem_temps, 1, s->nb_globals);
                 }
 
-                /* input args are live */
+                /* record arguments that die in this opcode */
                 for (i = nb_oargs; i < nb_oargs + nb_iargs; i++) {
                     arg = args[i];
                     if (dead_temps[arg]) {
                         dead_args |= (1 << i);
                     }
+                }
+                /* input arguments are live for preceeding opcodes */
+                for (i = nb_oargs; i < nb_oargs + nb_iargs; i++) {
+                    arg = args[i];
                     dead_temps[arg] = 0;
                 }
                 s->op_dead_args[oi] = dead_args;
-- 
2.1.4

Re: [Qemu-devel] [PATCH v2] tcg: fix dead computation for repeated input arguments

[Richard Henderson]

On 05/21/2015 12:35 PM, Aurelien Jarno wrote:
> When the same temp is used twice or more as an input argument to a TCG
> instruction, the dead computation code doesn't recognize the second use
> as a dead temp. This is because the temp is marked as live in the same
> loop where dead inputs are checked.
> 
> The fix is to split the loop in two parts. This avoid emitting a move
> and using a register for the movcond instruction when used as "move if
> true" on x86-64. This might bring more improvements on RISC TCG targets
> which don't have outputs aliased to inputs.
> 
> Cc: Richard Henderson <rth@twiddle.net>
> Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
> ---
>  tcg/tcg.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)

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


r~