[RFC PATCH] tcg/optimize: only read val after const check

[RFC PATCH] tcg/optimize: only read val after const check

[Alex Bennée]

valgrind pointed out that arg_info()->val can be undefined which will
be the case if the arguments are not constant. The ordering of the
checks will have ensured we never relied on an undefined value but for
the sake of completeness re-order the code to be clear.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
---
 tcg/optimize.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/tcg/optimize.c b/tcg/optimize.c
index e573000951..06213fd434 100644
--- a/tcg/optimize.c
+++ b/tcg/optimize.c
@@ -552,10 +552,10 @@ static bool do_constant_folding_cond_eq(TCGCond c)
 static int do_constant_folding_cond(TCGType type, TCGArg x,
                                     TCGArg y, TCGCond c)
 {
-    uint64_t xv = arg_info(x)->val;
-    uint64_t yv = arg_info(y)->val;
-
     if (arg_is_const(x) && arg_is_const(y)) {
+        uint64_t xv = arg_info(x)->val;
+        uint64_t yv = arg_info(y)->val;
+
         switch (type) {
         case TCG_TYPE_I32:
             return do_constant_folding_cond_32(xv, yv, c);
@@ -567,7 +567,7 @@ static int do_constant_folding_cond(TCGType type, TCGArg x,
         }
     } else if (args_are_copies(x, y)) {
         return do_constant_folding_cond_eq(c);
-    } else if (arg_is_const(y) && yv == 0) {
+    } else if (arg_is_const(y) && arg_info(y)->val == 0) {
         switch (c) {
         case TCG_COND_LTU:
             return 0;
-- 
2.30.2

Re: [RFC PATCH] tcg/optimize: only read val after const check

[Philippe Mathieu-Daudé via]

On 9/2/22 12:21, Alex Bennée wrote:
> valgrind pointed out that arg_info()->val can be undefined which will
> be the case if the arguments are not constant. The ordering of the
> checks will have ensured we never relied on an undefined value but for
> the sake of completeness re-order the code to be clear.
> 
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> ---
>   tcg/optimize.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

Re: [RFC PATCH] tcg/optimize: only read val after const check

[Richard Henderson]

On 2/9/22 01:21, Alex Bennée wrote:
> valgrind pointed out that arg_info()->val can be undefined which will
> be the case if the arguments are not constant. The ordering of the
> checks will have ensured we never relied on an undefined value but for
> the sake of completeness re-order the code to be clear.
> 
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> ---
>   tcg/optimize.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)

Thanks for this.  Just missed today's pull, but applied, finally.


r~

> 
> diff --git a/tcg/optimize.c b/tcg/optimize.c
> index e573000951..06213fd434 100644
> --- a/tcg/optimize.c
> +++ b/tcg/optimize.c
> @@ -552,10 +552,10 @@ static bool do_constant_folding_cond_eq(TCGCond c)
>   static int do_constant_folding_cond(TCGType type, TCGArg x,
>                                       TCGArg y, TCGCond c)
>   {
> -    uint64_t xv = arg_info(x)->val;
> -    uint64_t yv = arg_info(y)->val;
> -
>       if (arg_is_const(x) && arg_is_const(y)) {
> +        uint64_t xv = arg_info(x)->val;
> +        uint64_t yv = arg_info(y)->val;
> +
>           switch (type) {
>           case TCG_TYPE_I32:
>               return do_constant_folding_cond_32(xv, yv, c);
> @@ -567,7 +567,7 @@ static int do_constant_folding_cond(TCGType type, TCGArg x,
>           }
>       } else if (args_are_copies(x, y)) {
>           return do_constant_folding_cond_eq(c);
> -    } else if (arg_is_const(y) && yv == 0) {
> +    } else if (arg_is_const(y) && arg_info(y)->val == 0) {
>           switch (c) {
>           case TCG_COND_LTU:
>               return 0;