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[trunk] Addition to subreg section of rtl.text.
- From: Kenneth Zadeck <zadeck at naturalbridge dot com>
- To: gcc-patches <gcc-patches at gcc dot gnu dot org>, Ian Lance Taylor <iant at google dot com>, "Bonzini, Paolo" <bonzini at gnu dot org>, Richard Sandiford <rsandifo at nildram dot co dot uk>
- Date: Fri, 14 Mar 2008 10:22:54 -0400
- Subject: [trunk] Addition to subreg section of rtl.text.
Does every one agree that what i am adding is correct?
kenny
Index: rtl.texi
===================================================================
--- rtl.texi (revision 133159)
+++ rtl.texi (working copy)
@@ -1730,15 +1730,21 @@ are in @var{m}.
Sometimes @var{m} is wider than the mode of @var{reg}. These
@code{subreg} expressions are often called @dfn{paradoxical}. They are
used in cases where we want to refer to an object in a wider mode but do
-not care what value the additional bits have. The reload pass ensures
-that paradoxical references are only made to hard registers.
-
+not care what value the additional bits have. The smaller register
+always overlaps the least significant bits of the larger register and
+the @var{bytenum} is always zero for paradoxical registers (even on big
+endian machines). The reload pass ensures that paradoxical references
+are only made to hard registers.
+
The other use of @code{subreg} is to extract the individual registers of
a multi-register value. Machine modes such as @code{DImode} and
@code{TImode} can indicate values longer than a word, values which
usually require two or more consecutive registers. To access one of the
registers, use a @code{subreg} with mode @code{SImode} and a
-@var{bytenum} offset that says which register.
+@var{bytenum} offset that says which register. In this case, the
+@var(bytenum) must align the outer value to a word boundary if the inner
+register is a psuedo or to a register boundary if the inner register is
+a hard register.
Storing in a non-paradoxical @code{subreg} has undefined results for
bits belonging to the same word as the @code{subreg}. This laxity makes