Re: tcg: pointer size warning on x32 arch

[Philippe Mathieu-Daudé]

On 9/11/21 11:06 PM, Philippe Mathieu-Daudé wrote:
> On 9/11/21 7:50 PM, Michael Tokarev wrote:
>> Hi.
>>
>> The following warning is reported by the C compiler when compiling
>> tcg code on x32 architecture:
>>
>> In file included from ../../tcg/tcg.c:429:
>> tcg/i386/tcg-target.c.inc: In function ‘tcg_out_movi_int’:
>> tcg/i386/tcg-target.c.inc:959:30: warning: cast to pointer from integer
>> of different size [-Wint-to-pointer-cast]
>>   959 |     diff = tcg_pcrel_diff(s, (const void *)arg) - 7;
> 
> Likely fixed by:
> 
> ---
> diff --git a/tcg/i386/tcg-target.c.inc b/tcg/i386/tcg-target.c.inc
> index 98d924b91a8..0895f5670a1 100644
> --- a/tcg/i386/tcg-target.c.inc
> +++ b/tcg/i386/tcg-target.c.inc
> @@ -956,7 +956,7 @@ static void tcg_out_movi_int(TCGContext *s, TCGType
> type,
>      }
> 
>      /* Try a 7 byte pc-relative lea before the 10 byte movq.  */
> -    diff = tcg_pcrel_diff(s, (const void *)arg) - 7;
> +    diff = tcg_pcrel_diff(s, (const void *)(uintptr_t)arg) - 7;

Hmm not quite. At this point tcg_out_movi_int() already checked 'arg'
does not fit into a 32-bit value... And on x32 we have sizeof(void*) = 4
so we can't cast a >32-bit value that way.

But tcg_out_movi_int() is called by tcg_out_movi(), and all 'arg' values
are either 0, 1 or a host address (often casted as uintptr_t). So on x32
this value is already truncated (safe enough?). Thus this code seems
unlikely reached there (with a >32-bit value).

So maybe this is sufficient to silent the warning?

---
diff --git a/tcg/i386/tcg-target.c.inc b/tcg/i386/tcg-target.c.inc
index 98d924b91a8..2e6304f26b1 100644
--- a/tcg/i386/tcg-target.c.inc
+++ b/tcg/i386/tcg-target.c.inc
@@ -955,6 +955,8 @@ static void tcg_out_movi_int(TCGContext *s, TCGType
type,
         return;
     }

+    assert(sizeof(uintptr_t) > sizeof(uint32_t));
+
     /* Try a 7 byte pc-relative lea before the 10 byte movq.  */
     diff = tcg_pcrel_diff(s, (const void *)arg) - 7;
     if (diff == (int32_t)diff) {
---