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Diffstat (limited to 'gcc/doc/rtl.texi')
| -rw-r--r-- | gcc/doc/rtl.texi | 9 |
1 files changed, 9 insertions, 0 deletions
diff --git a/gcc/doc/rtl.texi b/gcc/doc/rtl.texi index 66351dd57c9..f6e00deeaa5 100644 --- a/gcc/doc/rtl.texi +++ b/gcc/doc/rtl.texi @@ -2157,6 +2157,15 @@ TARGET_CAN_CHANGE_MODE_CLASS (@var{m2}, @var{m1}, @var{class}) must be false for every class @var{class} that includes @var{reg}. +GCC must be able to determine at compile time whether a subreg is +paradoxical, whether it occupies a whole number of blocks, or whether +it is a lowpart of a block. This means that certain combinations of +variable-sized mode are not permitted. For example, if @var{m2} +holds @var{n} @code{SI} values, where @var{n} is greater than zero, +it is not possible to form a @code{DI} @code{subreg} of it; such a +@code{subreg} would be paradoxical when @var{n} is 1 but not when +@var{n} is greater than 1. + @findex SUBREG_REG @findex SUBREG_BYTE The first operand of a @code{subreg} expression is customarily accessed |