forked from OSchip/llvm-project
70 lines
2.5 KiB
ReStructuredText
70 lines
2.5 KiB
ReStructuredText
Redirectors
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===========
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When implementing a new C standard library (referred to as *libc* henceforth in
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this document) starting from scratch, it is unrealistic to expect that we will
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have the entire library available from day one. In such a scenario, a practical
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approach is to redirect calls to the unimplemented functions to the same
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functions from another fully functional libc implementation. Such a scheme can
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also serve users who would like to mix and match implementations from LLVM libc
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and another libc implementation. On most platforms, this other libc can be the
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system libc itself. In this document, we present a strategy one can employ to
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build redirectors to redirect from LLVM libc to the system libc. For now, the
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scheme presented is limited to ELF platforms.
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Highlevel Mechanism
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-------------------
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The highlevel scheme is as below:
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<img src="./redirectors_schematic.svg">
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As shown in the diagram, the mechanism involves a redirector dynamic library
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which goes in between the llvm-libc static library and the system libc dynamic
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library. Essentially, LLVM libc provides implementations for all public
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functions. However, some of the implementations do not actually implement the
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expected functionality. Instead, they just call the corresponding function in
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the redirector library, which in turn calls the same function from the system
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libc.
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Implementation of redirecting entrypoints
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-----------------------------------------
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Let us take the ``round`` function from ``math.h`` as an example to see what
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it's implementation looks like when it just redirects to the ``round`` function
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from the system libc.
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::
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namespace llvm_libc {
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double __redirected_round(double);
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double LLVM_LIBC_ENTRYPOINT(round)(double x) {
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return __redirected_round(x);
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}
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} // namespace llvm_libc
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As can be seen, the ``round`` function from LLVM libc does not call the
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``round`` function from the system libc directly. It calls a function
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``__redirected_round`` from the redirector library. The rest of the
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code follows the conventions described in the *implementation standard*
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document.
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Implementation of the redirector function
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-----------------------------------------
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The function ``__redirected_round`` calls the ``round`` function from the system
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libc. Its implementation is as follows::
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#include <math.h> // Header file from the system libc
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namespace llvm_libc {
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double __redirected_round(double x) {
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return ::round(x); // Call to round from the system libc
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}
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} // namespace llvm_libc
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