Fri Dec 4 20:15:57 1998 Bernd Schmidt <crux@pool.informatik.rwth-aachen.de>
+ * tm.texi (SMALL_REGISTER_CLASSES): Make description match reality.
+
* final.c (cleanup_subreg_operands): Delete some unused code.
* recog.h (MAX_RECOG_ALTERNATIVES): New macro.
@findex SMALL_REGISTER_CLASSES
@item SMALL_REGISTER_CLASSES
-Normally the compiler avoids choosing registers that have been
-explicitly mentioned in the rtl as spill registers (these registers are
-normally those used to pass parameters and return values). However,
-some machines have so few registers of certain classes that there
-would not be enough registers to use as spill registers if this were
-done.
+On some machines, it is risky to let hard registers live across arbitrary
+insns. Typically, these machines have instructions that require values
+to be in specific registers (like an accumulator), and reload will fail
+if the required hard register is used for another purpose across such an
+insn.
Define @code{SMALL_REGISTER_CLASSES} to be an expression with a non-zero
value on these machines. When this macro has a non-zero value, the
-compiler allows registers explicitly used in the rtl to be used as spill
-registers but avoids extending the lifetime of these registers.
+compiler will try to minimize the lifetime of hard registers.
It is always safe to define this macro with a non-zero value, but if you
unnecessarily define it, you will reduce the amount of optimizations
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