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Re: [PATCH 2/4] target/m68k: increase size of m68k CPU features from uin


From: BALATON Zoltan
Subject: Re: [PATCH 2/4] target/m68k: increase size of m68k CPU features from uint32_t to uint64_t
Date: Tue, 20 Sep 2022 21:01:20 +0200 (CEST)

On Tue, 20 Sep 2022, Mark Cave-Ayland wrote:
On 17/09/2022 23:27, Philippe Mathieu-Daudé via wrote:

On 17/9/22 14:09, BALATON Zoltan wrote:
On Sat, 17 Sep 2022, Mark Cave-Ayland wrote:
There are already 32 feature bits in use, so change the size of the m68k
CPU features to uint64_t (allong with the associated m68k_feature()
functions) to allow up to 64 feature bits to be used.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
---
target/m68k/cpu.c | 4 ++--
target/m68k/cpu.h | 6 +++---
2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/target/m68k/cpu.c b/target/m68k/cpu.c
index f681be3a2a..7b4797e2f1 100644
--- a/target/m68k/cpu.c
+++ b/target/m68k/cpu.c
@@ -38,12 +38,12 @@ static bool m68k_cpu_has_work(CPUState *cs)

static void m68k_set_feature(CPUM68KState *env, int feature)
{
-    env->features |= (1u << feature);
+    env->features |= (1ul << feature);

         env->features = deposit64(env->features, feature, 1, 1);

}

static void m68k_unset_feature(CPUM68KState *env, int feature)
{
-    env->features &= (-1u - (1u << feature));
+    env->features &= (-1ul - (1ul << feature));

         env->features = deposit64(env->features, feature, 1, 0);

Should these be ull instead of ul?

Yes. Not needed if using the <qemu/bitops.h> extract/deposit API.

I must admit I find the deposit64() variants not particularly easy to read:

I agree with that and also dislike the dposit/extract functions that would not bring much here. Maybe they are useful for multiple bits but for a single bit they just add overhead and obfuscation.

if we're considering alterations rather than changing the constant suffix then I'd much rather go for:

   env->features |= (1ULL << feature);

and:

   env->features &= ~(1ULL << feature);

There's also a BIT_ULL macro which could be used but it's up to you, shifting 1ULL is also simple enough to read.

Regards,
BALATON Zoltan

Laurent, what would be your preference?

}

static void m68k_cpu_reset(DeviceState *dev)
diff --git a/target/m68k/cpu.h b/target/m68k/cpu.h
index 67b6c12c28..d3384e5d98 100644
--- a/target/m68k/cpu.h
+++ b/target/m68k/cpu.h
@@ -154,7 +154,7 @@ typedef struct CPUArchState {
    struct {} end_reset_fields;

    /* Fields from here on are preserved across CPU reset. */
-    uint32_t features;
+    uint64_t features;
} CPUM68KState;

/*
@@ -539,9 +539,9 @@ enum m68k_features {
    M68K_FEATURE_TRAPCC,
};

-static inline int m68k_feature(CPUM68KState *env, int feature)
+static inline uint64_t m68k_feature(CPUM68KState *env, int feature)

Why uint64_t? Can we simplify using a boolean?

I don't really feel strongly either way here. Again I'm happy to go with whatever Laurent would prefer as maintainer.

{
-    return (env->features & (1u << feature)) != 0;
+    return (env->features & (1ul << feature)) != 0;

         return extract64(env->features, feature, 1) == 1;

}

void m68k_cpu_list(void);


ATB,

Mark.


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