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1 @c Copyright (C) 1988-2020 Free Software Foundation, Inc.
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2 @c This is part of the GCC manual.
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3 @c For copying conditions, see the file gcc.texi.
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4
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5 @node Interface
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6 @chapter Interfacing to GCC Output
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7 @cindex interfacing to GCC output
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8 @cindex run-time conventions
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9 @cindex function call conventions
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10 @cindex conventions, run-time
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11
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12 GCC is normally configured to use the same function calling convention
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13 normally in use on the target system. This is done with the
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14 machine-description macros described (@pxref{Target Macros}).
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15
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16 @cindex unions, returning
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17 @cindex structures, returning
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18 @cindex returning structures and unions
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19 However, returning of structure and union values is done differently on
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20 some target machines. As a result, functions compiled with PCC
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21 returning such types cannot be called from code compiled with GCC,
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22 and vice versa. This does not cause trouble often because few Unix
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23 library routines return structures or unions.
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24
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25 GCC code returns structures and unions that are 1, 2, 4 or 8 bytes
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26 long in the same registers used for @code{int} or @code{double} return
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27 values. (GCC typically allocates variables of such types in
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28 registers also.) Structures and unions of other sizes are returned by
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29 storing them into an address passed by the caller (usually in a
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30 register). The target hook @code{TARGET_STRUCT_VALUE_RTX}
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31 tells GCC where to pass this address.
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32
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33 By contrast, PCC on most target machines returns structures and unions
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34 of any size by copying the data into an area of static storage, and then
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35 returning the address of that storage as if it were a pointer value.
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36 The caller must copy the data from that memory area to the place where
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37 the value is wanted. This is slower than the method used by GCC, and
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38 fails to be reentrant.
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39
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40 On some target machines, such as RISC machines and the 80386, the
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41 standard system convention is to pass to the subroutine the address of
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42 where to return the value. On these machines, GCC has been
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43 configured to be compatible with the standard compiler, when this method
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44 is used. It may not be compatible for structures of 1, 2, 4 or 8 bytes.
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45
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46 @cindex argument passing
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47 @cindex passing arguments
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48 GCC uses the system's standard convention for passing arguments. On
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49 some machines, the first few arguments are passed in registers; in
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50 others, all are passed on the stack. It would be possible to use
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51 registers for argument passing on any machine, and this would probably
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52 result in a significant speedup. But the result would be complete
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53 incompatibility with code that follows the standard convention. So this
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54 change is practical only if you are switching to GCC as the sole C
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55 compiler for the system. We may implement register argument passing on
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56 certain machines once we have a complete GNU system so that we can
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57 compile the libraries with GCC@.
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58
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59 On some machines (particularly the SPARC), certain types of arguments
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60 are passed ``by invisible reference''. This means that the value is
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61 stored in memory, and the address of the memory location is passed to
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62 the subroutine.
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63
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64 @cindex @code{longjmp} and automatic variables
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65 If you use @code{longjmp}, beware of automatic variables. ISO C says that
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66 automatic variables that are not declared @code{volatile} have undefined
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67 values after a @code{longjmp}. And this is all GCC promises to do,
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68 because it is very difficult to restore register variables correctly, and
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69 one of GCC's features is that it can put variables in registers without
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70 your asking it to.
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