On Tue, Aug 13, 2013 at 09:43:23AM -0700, H. Peter Anvin wrote:
> On 08/13/2013 08:37 AM, Cyrill Gorcunov wrote:
> >>
> >> Does it actually matter, generated-code-wise, or is the compiler smart
> >> enough to figure it out? The reason I'm asking is because it makes the
> >
> > gcc-4.7.2 is smart enough to suppress useless masking (ie ((1u << 31) - 1))
> > completely but I don't know if this can be assumed for all gcc series.
> >
>
> I would be highly surprised if it wasn't the case for any gcc we care about.
Does below one looks better? (Btw, what about the snippet we have there as well
#if _PAGE_BIT_FILE < _PAGE_BIT_PROTNONE
#define PTE_FILE_SHIFT2 (_PAGE_BIT_FILE + 1)
#define PTE_FILE_SHIFT3 (_PAGE_BIT_PROTNONE + 1)
#else
#define PTE_FILE_SHIFT2 (_PAGE_BIT_PROTNONE + 1)
#define PTE_FILE_SHIFT3 (_PAGE_BIT_FILE + 1)
#endif
where
#define _PAGE_BIT_PROTNONE _PAGE_BIT_GLOBAL -> 8
#define _PAGE_BIT_FILE _PAGE_BIT_DIRTY -> 6
so I wonder where the cases on x86 when _PAGE_BIT_FILE > _PAGE_BIT_PROTNONE,
what i'm missing here?)
---
arch/x86/include/asm/pgtable-2level.h | 37 +++++++++++++++++++---------------
1 file changed, 21 insertions(+), 16 deletions(-)
Index: linux-2.6.git/arch/x86/include/asm/pgtable-2level.h
===================================================================
--- linux-2.6.git.orig/arch/x86/include/asm/pgtable-2level.h
+++ linux-2.6.git/arch/x86/include/asm/pgtable-2level.h
@@ -55,8 +55,11 @@ static inline pmd_t native_pmdp_get_and_
#define native_pmdp_get_and_clear(xp) native_local_pmdp_get_and_clear(xp)
#endif
-#define _mfrob(v,r,m,l) ((((v) >> (r)) & (m)) << (l))
-#define __frob(v,r,l) (((v) >> (r)) << (l))
+/*
+ * For readable bitfield manipulations.
+ */
+#define PTE_FILE_NOMASK (-1U)
+#define __bfop(v,r,m,l) ((((v) >> (r)) & (m)) << (l))
#ifdef CONFIG_MEM_SOFT_DIRTY
@@ -83,17 +86,17 @@ static inline pmd_t native_pmdp_get_and_
#define PTE_FILE_LSHIFT4 (PTE_FILE_BITS1 + PTE_FILE_BITS2 + PTE_FILE_BITS3)
#define pte_to_pgoff(pte) \
- (_mfrob((pte).pte_low, PTE_FILE_SHIFT1, PTE_FILE_MASK1, 0) + \
- _mfrob((pte).pte_low, PTE_FILE_SHIFT2, PTE_FILE_MASK2, PTE_FILE_LSHIFT2) + \
- _mfrob((pte).pte_low, PTE_FILE_SHIFT3, PTE_FILE_MASK3, PTE_FILE_LSHIFT3) + \
- __frob((pte).pte_low, PTE_FILE_SHIFT4, PTE_FILE_LSHIFT4))
+ (__bfop((pte).pte_low, PTE_FILE_SHIFT1, PTE_FILE_MASK1, 0) + \
+ __bfop((pte).pte_low, PTE_FILE_SHIFT2, PTE_FILE_MASK2, PTE_FILE_LSHIFT2) + \
+ __bfop((pte).pte_low, PTE_FILE_SHIFT3, PTE_FILE_MASK3, PTE_FILE_LSHIFT3) + \
+ __bfop((pte).pte_low, PTE_FILE_SHIFT4, PTE_FILE_NOMASK, PTE_FILE_LSHIFT4))
#define pgoff_to_pte(off) \
((pte_t) { .pte_low = \
- _mfrob(off, 0, PTE_FILE_MASK1, PTE_FILE_SHIFT1) + \
- _mfrob(off, PTE_FILE_LSHIFT2, PTE_FILE_MASK2, PTE_FILE_SHIFT2) + \
- _mfrob(off, PTE_FILE_LSHIFT3, PTE_FILE_MASK3, PTE_FILE_SHIFT3) + \
- __frob(off, PTE_FILE_LSHIFT4, PTE_FILE_SHIFT4) + \
+ __bfop(off, 0, PTE_FILE_MASK1, PTE_FILE_SHIFT1) + \
+ __bfop(off, PTE_FILE_LSHIFT2, PTE_FILE_MASK2, PTE_FILE_SHIFT2) + \
+ __bfop(off, PTE_FILE_LSHIFT3, PTE_FILE_MASK3, PTE_FILE_SHIFT3) + \
+ __bfop(off, PTE_FILE_LSHIFT4, PTE_FILE_NOMASK, PTE_FILE_SHIFT4) + \
_PAGE_FILE })
#else /* CONFIG_MEM_SOFT_DIRTY */
@@ -121,19 +124,21 @@ static inline pmd_t native_pmdp_get_and_
#define PTE_FILE_LSHIFT3 (PTE_FILE_BITS1 + PTE_FILE_BITS2)
#define pte_to_pgoff(pte) \
- (_mfrob((pte).pte_low, PTE_FILE_SHIFT1, PTE_FILE_MASK1, 0) + \
- _mfrob((pte).pte_low, PTE_FILE_SHIFT2, PTE_FILE_MASK2, PTE_FILE_LSHIFT2) + \
- __frob((pte).pte_low, PTE_FILE_SHIFT3, PTE_FILE_LSHIFT3))
+ (__bfop((pte).pte_low, PTE_FILE_SHIFT1, PTE_FILE_MASK1, 0) + \
+ __bfop((pte).pte_low, PTE_FILE_SHIFT2, PTE_FILE_MASK2, PTE_FILE_LSHIFT2) + \
+ __bfop((pte).pte_low, PTE_FILE_SHIFT3, PTE_FILE_NOMASK, PTE_FILE_LSHIFT3))
#define pgoff_to_pte(off) \
((pte_t) { .pte_low = \
- _mfrob(off, 0, PTE_FILE_MASK1, PTE_FILE_SHIFT1) + \
- _mfrob(off, PTE_FILE_LSHIFT2, PTE_FILE_MASK2, PTE_FILE_SHIFT2) + \
- __frob(off, PTE_FILE_LSHIFT3, PTE_FILE_SHIFT3) + \
+ __bfop(off, 0, PTE_FILE_MASK1, PTE_FILE_SHIFT1) + \
+ __bfop(off, PTE_FILE_LSHIFT2, PTE_FILE_MASK2, PTE_FILE_SHIFT2) + \
+ __bfop(off, PTE_FILE_LSHIFT3, PTE_FILE_NOMASK, PTE_FILE_SHIFT3) + \
_PAGE_FILE })
#endif /* CONFIG_MEM_SOFT_DIRTY */
+#undef __bfop
+
/* Encode and de-code a swap entry */
#if _PAGE_BIT_FILE < _PAGE_BIT_PROTNONE
#define SWP_TYPE_BITS (_PAGE_BIT_FILE - _PAGE_BIT_PRESENT - 1)
--
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