You cannot remove sizeof(A) from the expression, as doing so may cause the updated expression to compute a different answer.
Consider ((H*)var)->size = 64, B = 32, and A = 64. In the original form, you have the expression (64 + 64 - 1) / 64 * 64 / 32 which equals 2. In your proposal, you have the expression (64 + 64 - 1) / 32 which equals 3. This could result in incorrect behavior later on. If I were Daniel, I would want to see something that shows the change in behavior does not result in incorrect behavior. Regards, Nicholas Vinson On 4/11/19 1:39 AM, Milo Wenxiang Niu wrote: > From: Milo Wenxiang X Niu <printer...@aliyun.com> > > It's just to remove the common factor: "sizeof (grub_addr_t)" from the > numerator and denominator of the fractional expression of next var. > Let me explain it: > Shortly: > H: struct grub_module_header ; > B: grub_uint32_t ; > A: grub_addr_t; > > Thus, original expression can be expressed as: > var = (H *)((B*)var) + ( offset_exp )) > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > (((H*)var)->size + sizeof(A) - 1) sizeof(A) | > offset_exp = --------------------------------- * ----------- | > sizeof(A) sizeof(B) | > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > Remove the common factor "sizeof(A)" of fractional offset_exp, we got the > result. > offset_exp_new = ((H*)->size + sizeof(A) - 1) / sizeof(B) > = ((struct grub_module_header *) var)->size + sizeof > (grub_addr_t) -1) / sizeof (grub_uint32_t) > > so: > var =(H *)(((B*)var) + ( (((H*)var)->size + sizeof(A) - 1) / sizeof(B) )) > That's what I do. > > Still, the new offset express is meaningfull: > *numerator: ((struct grub_module_header *) var)->size + sizeof > (grub_addr_t) -1) > it present the offset value united by byte. > *denominator: sizeof (grub_uint32_t) > it's means "struct grub_module_header" aligned by > sizeof(grub_uint32_t) > --- > include/grub/kernel.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/include/grub/kernel.h b/include/grub/kernel.h > index 133a37c..b257224 100644 > --- a/include/grub/kernel.h > +++ b/include/grub/kernel.h > @@ -104,7 +104,7 @@ extern grub_addr_t EXPORT_VAR (grub_modbase); > var && (grub_addr_t) var \ > < (grub_modbase + (((struct grub_module_info *) grub_modbase)->size)); > \ > var = (struct grub_module_header *) > \ > - (((grub_uint32_t *) var) + ((((struct grub_module_header *) var)->size + > sizeof (grub_addr_t) - 1) / sizeof (grub_addr_t)) * (sizeof (grub_addr_t) / > sizeof (grub_uint32_t)))) > + (((grub_uint32_t *) var) + ((((struct grub_module_header *) var)->size + > sizeof (grub_addr_t) - 1) / sizeof (grub_uint32_t)))) > > grub_addr_t grub_modules_get_end (void); > >
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