Comments on: Linkers part 4 http://www.airs.com/blog/archives/41 Ian Lance Taylor Wed, 8 Oct 2008 05:55:28 +0000 http://wordpress.org/?v=1.5.2 Comment on Linkers part 4 by: Ian Lance Taylor http://www.airs.com/blog/archives/41#comment-4286 Fri, 14 Sep 2007 04:32:17 +0000 http://www.airs.com/blog/archives/41#comment-4286 Thanks--I remembered to put that bit into part 6. Thanks–I remembered to put that bit into part 6.

]]> Comment on Linkers part 4 by: jrlevine http://www.airs.com/blog/archives/41#comment-4273 Thu, 13 Sep 2007 23:46:33 +0000 http://www.airs.com/blog/archives/41#comment-4273 The other advantage of PIC is better code sharing. If there's no relocation fixups in a page, all processes can share the same physical copy of the page. As soon as there's load time fixup, you need a separate copy of the page per process. Making and maintaining the copy is way more work than the fixups themselves, since it requires a trap to the system and a copy of the whole page. The other advantage of PIC is better code sharing. If there’s no relocation fixups in a page, all processes can share the same physical copy of the page. As soon as there’s load time fixup, you need a separate copy of the page per process. Making and maintaining the copy is way more work than the fixups themselves, since it requires a trap to the system and a copy of the whole page.

]]> Comment on Linkers part 4 by: Mark J. Wielaard » Ian Lance Taylor’s Linker Notes http://www.airs.com/blog/archives/41#comment-4026 Fri, 31 Aug 2007 16:21:17 +0000 http://www.airs.com/blog/archives/41#comment-4026 [...] Linkers part 4 - Shared Libraries (Procedure Linkage Table - PLT and Global Offset Table - GOT). [...] […] Linkers part 4 - Shared Libraries (Procedure Linkage Table - PLT and Global Offset Table - GOT). […]

]]> Comment on Linkers part 4 by: Ian Lance Taylor http://www.airs.com/blog/archives/41#comment-4000 Thu, 30 Aug 2007 04:02:55 +0000 http://www.airs.com/blog/archives/41#comment-4000 %ebx is a callee saved register for the i386, which means that if a function changes %ebx, it must save it at the start of a function and restore it at the end. This is normally the best choice for the GOT register, since it means that the the value does not have to be recomputed or restored after a function call. However, if a function does not call any other functions (i.e., it is a leaf function), then it is not important to keep the address of the GOT in a callee saved register. In fact, in that case, it is better to keep it in a caller saved register--that is, a register which a function is permitted to change without needing to save and restore it. So gcc optimizes by putting the GOT table in a caller saved register in a leaf function. gcc does not always use %ecx, incidentally, though that is a common choice. Depending on the function, it may choose any available caller saved register. %ebx is a callee saved register for the i386, which means that if a function changes %ebx, it must save it at the start of a function and restore it at the end. This is normally the best choice for the GOT register, since it means that the the value does not have to be recomputed or restored after a function call.

However, if a function does not call any other functions (i.e., it is a leaf function), then it is not important to keep the address of the GOT in a callee saved register. In fact, in that case, it is better to keep it in a caller saved register–that is, a register which a function is permitted to change without needing to save and restore it. So gcc optimizes by putting the GOT table in a caller saved register in a leaf function.

gcc does not always use %ecx, incidentally, though that is a common choice. Depending on the function, it may choose any available caller saved register.

]]> Comment on Linkers part 4 by: wh5a http://www.airs.com/blog/archives/41#comment-3993 Wed, 29 Aug 2007 08:57:26 +0000 http://www.airs.com/blog/archives/41#comment-3993 Thanks for your great article. I've got one question: It seems if a pic function only access global variables but does not call global functions, it will call __i686.get_pc_thunk.cx to compute the GOT address, and its value will be cached in %ecx, instead of %ebx. Why is that? I'm running Linux. Thanks. Thanks for your great article. I’ve got one question:

It seems if a pic function only access global variables but does not call global functions, it will call __i686.get_pc_thunk.cx to compute the GOT address, and its value will be cached in %ecx, instead of %ebx. Why is that?

I’m running Linux. Thanks.

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