Shared library woes and the price of PIC

It recently came to my attention that the GNU linker on ARM lacks support for several relocation types in shared libraries. Specifically, code using MOVW/MOVT instruction pairs to load the address of data symbols will not work in a shared library. The linker silently drops the necessary relocations, resulting in a runtime crash.

When I pointed out this shortcoming to Paul Brook of CodeSourcery, his response was that such relocations in shared libraries are not supported by the GNU tools, will never be, and that shared libraries should be built with position-independent code (PIC). This is an unfortunate attitude, and doubly so considering that the latest CodeSourcery GCC version will generate these instructions with default settings. In other words, the 2008q3 release of CodeSourcery GCC will, with default flags, build crashing shared libraries without so much as a warning.

The refusal to support non-PIC shared libraries is unfortunate also from a performance point of view. Position independent code is inherently slower than normal code.

In order to find out just how much slower PIC is on ARM, I made two builds of FFmpeg, one normal and one with PIC. The PIC build is about 1.7% slower in several tests, among them H.264 video decoding.

On typically resource-constrained ARM systems it would be nice to have the option of space-saving shared libraries without paying the PIC penalty in performance. Until now this option has been a reality. With CodeSourcery lazily refusing to support the relocations required by the latest version of their own compiler, this option may soon be a thing of the past, at least if the bugs that have haunted recent compiler releases are fixed in upcoming versions.

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