Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH 2/4] arm64: implement support for static call trampolines | Date | Mon, 25 Oct 2021 14:25:53 +0000 |
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From: Frederic Weisbecker > Sent: 25 October 2021 13:21 > > Implement arm64 support for the 'unoptimized' static call variety, which > routes all calls through a single trampoline that is patched to perform a > tail call to the selected function. > > It is expected that the direct branch instruction will be able to cover > the common case. However, given that static call targets may be located > in modules loaded out of direct branching range, we need a fallback path > that loads the address into R16 and uses a branch-to-register (BR) > instruction to perform an indirect call. > ... > +void arch_static_call_transform(void *site, void *tramp, void *func, bool tail) > +{ > + /* > + * -0x8 <literal> > + * 0x0 bti c <--- trampoline entry point > + * 0x4 <branch or nop> > + * 0x8 ldr x16, <literal> > + * 0xc cbz x16, 20 > + * 0x10 br x16 > + * 0x14 ret > + */
Since the 'ldr x16, <literal>' is just a 32bit constant (for a pc-relative load).
Can't you save a word by making offset 0x4 <branch or ldr x16, <literal>> ?
Or am I missing something?
David
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