accessed in the same way and in addition contain a field
@code{CALL_INSN_FUNCTION_USAGE}, which contains a list (chain of
@code{expr_list} expressions) containing @code{use} and @code{clobber}
-expressions that denote hard registers used or clobbered by the called
-function. A register specified in a @code{clobber} in this list is
-modified @emph{after} the execution of the @code{call_insn}, while a
-register in a @code{clobber} in the body of the @code{call_insn} is
-clobbered before the insn completes execution. @code{clobber}
-expressions in this list augment registers specified in
+expressions that denote hard registers and @code{MEM}s used or
+clobbered by the called function.
+
+A @code{MEM} generally points to a stack slots in which arguments passed
+to the libcall by reference (@pxref{Register Arguments,
+FUNCTION_ARG_PASS_BY_REFERENCE}) are stored. If the argument is
+caller-copied (@pxref{Register Arguments, FUNCTION_ARG_CALLEE_COPIES}),
+the stack slot will be mentioned in @code{CLOBBER} and @code{USE}
+entries; if it's callee-copied, only a @code{USE} will appear, and the
+@code{MEM} may point to addresses that are not stack slots. These
+@code{MEM}s are used only in libcalls, because, unlike regular function
+calls, @code{CONST_CALL}s (which libcalls generally are, @pxref{Flags,
+CONST_CALL_P}) aren't assumed to read and write all memory, so flow
+would consider the stores dead and remove them. Note that, since a
+libcall must never return values in memory (@pxref{Aggregate Return,
+RETURN_IN_MEMORY}), there will never be a @code{CLOBBER} for a memory
+address holding a return value.
+
+@code{CLOBBER}ed registers in this list augment registers specified in
@code{CALL_USED_REGISTERS} (@pxref{Register Basics}).
@findex code_label
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