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doc: Update documentation on MODE_PARTIAL_INT subregs

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In d8487c949a, MODE_PARTIAL_INT modes were changed from having an
unknown number of undefined bits, to having a known number of undefined
bits, however the documentation on using SUBREG expressions with
MODE_PARTIAL_INT modes was not updated to reflect this.

gcc/ChangeLog:

	* doc/rtl.texi (subreg): Fix documentation to state there is a known
	number of undefined bits in regs and subregs of MODE_PARTIAL_INT modes.
This commit is contained in:
Jozef Lawrynowicz 2020-09-07 17:52:04 +01:00
parent 7f87e44669
commit 016b190036
1 changed files with 18 additions and 12 deletions
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30
gcc/doc/rtl.texi
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@ -2209,17 +2209,22 @@ whether the subreg is a lowpart of a block.
@end table
A @code{MODE_PARTIAL_INT} mode behaves as if it were as wide as the
corresponding @code{MODE_INT} mode, except that it has an unknown
number of undefined bits. For example:
corresponding @code{MODE_INT} mode, except that it has a number of
undefined bits, which are determined by the precision of the
mode.
For example, on a little-endian target which defines @code{PSImode}
to have a precision of 20 bits:
@smallexample
(subreg:PSI (reg:SI 0) 0)
@end smallexample
accesses the low 20 bits of @samp{(reg:SI 0)}.
@findex REGMODE_NATURAL_SIZE
accesses the whole of @samp{(reg:SI 0)}, but the exact relationship
between the @code{PSImode} value and the @code{SImode} value is not
defined. If we assume @samp{REGMODE_NATURAL_SIZE (DImode) <= 4},
Continuing with a @code{PSImode} precision of 20 bits, if we assume
@samp{REGMODE_NATURAL_SIZE (DImode) <= 4},
then the following two @code{subreg}s:
@smallexample
@ -2227,9 +2232,8 @@ then the following two @code{subreg}s:
(subreg:PSI (reg:DI 0) 4)
@end smallexample
represent independent 4-byte accesses to the two halves of
@samp{(reg:DI 0)}. Both @code{subreg}s have an unknown number
of undefined bits.
represent accesses to the low 20 bits of the two halves of
@samp{(reg:DI 0)}.
If @samp{REGMODE_NATURAL_SIZE (PSImode) <= 2} then these two @code{subreg}s:
@ -2240,15 +2244,17 @@ If @samp{REGMODE_NATURAL_SIZE (PSImode) <= 2} then these two @code{subreg}s:
represent independent 2-byte accesses that together span the whole
of @samp{(reg:PSI 0)}. Storing to the first @code{subreg} does not
affect the value of the second, and vice versa. @samp{(reg:PSI 0)}
has an unknown number of undefined bits, so the assignment:
affect the value of the second, and vice versa, so the assignment:
@smallexample
(set (subreg:HI (reg:PSI 0) 0) (reg:HI 4))
@end smallexample
does not guarantee that @samp{(subreg:HI (reg:PSI 0) 0)} has the
value @samp{(reg:HI 4)}.
sets the low 16 bits of @samp{(reg:PSI 0)} to @samp{(reg:HI 4)}, and
the high 4 defined bits of @samp{(reg:PSI 0)} retain their
original value. The behavior here is the same as for
normal @code{subreg}s, when there are no
@code{MODE_PARTIAL_INT} modes involved.
@cindex @code{TARGET_CAN_CHANGE_MODE_CLASS} and subreg semantics
The rules above apply to both pseudo @var{reg}s and hard @var{reg}s.