[PATCH] target/arm/translate.c: Delete/amend incorrect comments

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[PATCH] target/arm/translate.c: Delete/amend incorrect comments

Peter Maydell-5
In arm_tr_init_disas_context() we have a FIXME comment that suggests
"cpu_M0 can probably be the same as cpu_V0".  This isn't in fact
possible: cpu_V0 is used as a temporary inside gen_iwmmxt_shift(),
and that function is called in various places where cpu_M0 contains a
live value (i.e.  between gen_op_iwmmxt_movq_M0_wRn() and
gen_op_iwmmxt_movq_wRn_M0() calls).  Remove the comment.

We also have a comment on the declarations of cpu_V0/V1/M0 which
claims they're "for efficiency".  This isn't true with modern TCG, so
replace this comment with one which notes that they're only used with
the iwmmxt decode.

Signed-off-by: Peter Maydell <[hidden email]>
---
In an ideal world we would clean up the iwmmxt decode to remove
cpu_V0, cpu_v1 and cpu_M0 entirely -- they're only used as
temporaries, and in a modern coding style we would just create and
dispose of more carefully scoped TCG temps as we needed them.
---
 target/arm/translate.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/target/arm/translate.c b/target/arm/translate.c
index 27bf6cd8b51..87235db4640 100644
--- a/target/arm/translate.c
+++ b/target/arm/translate.c
@@ -59,8 +59,9 @@
 #define IS_USER(s) (s->user)
 #endif
 
-/* We reuse the same 64-bit temporaries for efficiency.  */
+/* These are TCG temporaries used only by the legacy iwMMXt decoder */
 static TCGv_i64 cpu_V0, cpu_V1, cpu_M0;
+/* These are TCG globals which alias CPUARMState fields */
 static TCGv_i32 cpu_R[16];
 TCGv_i32 cpu_CF, cpu_NF, cpu_VF, cpu_ZF;
 TCGv_i64 cpu_exclusive_addr;
@@ -8552,7 +8553,6 @@ static void arm_tr_init_disas_context(DisasContextBase *dcbase, CPUState *cs)
 
     cpu_V0 = tcg_temp_new_i64();
     cpu_V1 = tcg_temp_new_i64();
-    /* FIXME: cpu_M0 can probably be the same as cpu_V0.  */
     cpu_M0 = tcg_temp_new_i64();
 }
 
--
2.20.1


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Re: [PATCH] target/arm/translate.c: Delete/amend incorrect comments

Richard Henderson-3
On 8/3/20 6:28 AM, Peter Maydell wrote:

> In arm_tr_init_disas_context() we have a FIXME comment that suggests
> "cpu_M0 can probably be the same as cpu_V0".  This isn't in fact
> possible: cpu_V0 is used as a temporary inside gen_iwmmxt_shift(),
> and that function is called in various places where cpu_M0 contains a
> live value (i.e.  between gen_op_iwmmxt_movq_M0_wRn() and
> gen_op_iwmmxt_movq_wRn_M0() calls).  Remove the comment.
>
> We also have a comment on the declarations of cpu_V0/V1/M0 which
> claims they're "for efficiency".  This isn't true with modern TCG, so
> replace this comment with one which notes that they're only used with
> the iwmmxt decode.
>
> Signed-off-by: Peter Maydell <[hidden email]>
> ---

Reviewed-by: Richard Henderson <[hidden email]>

r~