cmd/link/internal/ld: assign temporary addresses to per-package text
If trampolines may be required, the current text addressing second
pass resets all assigned addresses, before assigning addresses and
laying down trampolines in a linear fashion. However, this approach
means that intra-package calls are to a symbol that has not yet
been assigned an address, when the symbol is ahead of the current
function.
In the case of RISC-V the JAL instruction is limited to +/-1MiB.
As such, if a call is to a symbol with no address currently assigned,
we have to assume that a trampoline will be required. During the
relocation phase we can fix up and avoid trampolines in some cases,
however this results in unused trampolines that are still present
in the binary (since removing them would change text addresses).
In order to significantly reduce the number of unused trampolines,
assign temporary addresses to functions within the same package,
based on the maximum number of trampolines that may be required by
a function. This allows for better decisions to be made regarding
the requirement for intra-package trampolines, as we reset the
addressing for a function, assign its final address and lay down
any resulting trampolines.
This results in ~2,300 unused trampolines being removed from the
Go binary and ~5,600 unused trampolines being removed from the
compile binary, on linux/riscv64.
Change-Id: I8a9cf035dea82e1e1e66ae5b1093dce78e4ff0d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/349650
Run-TryBot: Joel Sing <joel@sing.id.au> Reviewed-by: Than McIntosh <thanm@google.com> Reviewed-by: Bryan Mills <bcmills@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>