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| -rw-r--r-- | gcc/ChangeLog | 2 | ||||
| -rw-r--r-- | gcc/rtl.texi | 28 |
2 files changed, 30 insertions, 0 deletions
diff --git a/gcc/ChangeLog b/gcc/ChangeLog index 288f380..794cf5d 100644 --- a/gcc/ChangeLog +++ b/gcc/ChangeLog @@ -2,6 +2,8 @@ * combine.c (combine_simplify_rtx): Try to simplify VEC_SELECT of a VEC_CONCAT. + * rtl.texi (description of USE): Add note about possible pitfalls + with this rtx. From Richard Henderson: * reload1.c (choose_reload_regs): Compute need_mode properly. diff --git a/gcc/rtl.texi b/gcc/rtl.texi index 247807a..17dea1a 100644 --- a/gcc/rtl.texi +++ b/gcc/rtl.texi @@ -2057,6 +2057,34 @@ it may not be apparent why this is so. Therefore, the compiler will not attempt to delete previous instructions whose only effect is to store a value in @var{x}. @var{x} must be a @code{reg} expression. +In some situations, it may be tempting to add a @code{use} of a +register in a @code{parallel} to describe a situation where the value +of a special register will modify the behaviour of the instruction. +An hypothetical example might be a pattern for an addition that can +either wrap around or use saturating addition depending on the value +of a special control register: + +@example +(parallel [(set (reg:SI 2) (unspec:SI [(reg:SI 3) (reg:SI 4)] 0)) + (use (reg:SI 1))]) +@end example + +@noindent + +This will not work, several of the optimizers only look at expressions +locally; it is very likely that if you have multiple insns with +identical inputs to the @code{unspec}, they will be optimized away even +if register 1 changes in between. + +This means that @code{use} can @emph{only} be used to describe +that the register is live. You should think twice before adding +@code{use} statements, more often you will want to use @code{unspec} +instead. The @code{use} RTX is most commonly useful to describe that +a fixed register is implicitly used in an insn. It is also safe to use +in patterns where the compiler knows for other reasons that the result +of the whole pattern is variable, such as @samp{movstr@var{m}} or +@samp{call} patterns. + During the reload phase, an insn that has a @code{use} as pattern can carry a reg_equal note. These @code{use} insns will be deleted before the reload phase exits. |