3 ms·

Your figures seem a little off there. Yes, RISC-V is a little asymmetric, with the AUIPC being able to subtract exactly 2 GB from the PC or add 2GB-4KB to it, a

Your figures seem a little off there. Yes, RISC-V is a little asymmetric, with the AUIPC being able to subtract exactly 2 GB from the PC or add 2GB-4KB to it, and then a jalr/lb/sb can subtract an additional 2 KB or add 2KB-1. But the AArch64 adrp is also asymmetric because the relative reach depends on where in the 4 KB page the original PC is. It's only symmetric if the PC is 4 KB aligned. If you're part way through the page then there is more -ve reach and less +ve reach. A couple of KB fuzziness in what is basically a 32 bit reach in a 64 bit address space is pretty much completely irrelevant in both cases.

brandmeyer 8y ago
ADRP operates on the 4kB page of the pc (by truncation), not the entire pc. RISC-V could have implicitly added 2k in auipc and balanced out the bias. But they didn't.
- brucehoult 8y ago
  I know how ADRP operates. It's symmetric about the truncated PC. It's not symmetric about the actual PC. As I said in the last post.
  - brandmeyer 8y ago
    I think you are misunderstanding the benefit of having symmetric reach by the page. On AArch64, you can define a 2 GB contiguous slice of address space, built up out of whatever page size you find convenient for your system and plant a relocatable binary into it, up to 2 GB of size. Any instruction anywhere in the last page can reach any address in the first page, and vice versa. In RISC-V, if you try to do the same thing, you'd find that while any instructions in the last page can reach any address in the first page with room to spare. But some instructions in the first page cannot reach portions of the last page. Sure, it doesn't matter most of the time. It isn't ever really an obstacle in practice for the feller writing application code for the platform. But the linker has to be aware of it in the 'medium' code model as a special case for just this particular platform. Somebody had to write that special case to work around the hardware.
    - brucehoult 8y ago
      The linker code to calculate the necessary auipc and remaining offset for a relocation and do something else when out of bounds, was written years ago, is two lines of code, and no doubt took less time than this conversation. I don't even know of any application that has 1 GB of code, let alone 2 GB minus 2 KB (2,147,481,600 bytes).

brandmeyer 8y ago

ADRP operates on the 4kB page of the pc (by truncation), not the entire pc. RISC-V could have implicitly added 2k in auipc and balanced out the bias. But they didn't.

brucehoult 8y ago
I know how ADRP operates. It's symmetric about the truncated PC. It's not symmetric about the actual PC. As I said in the last post.
- brandmeyer 8y ago
  I think you are misunderstanding the benefit of having symmetric reach by the page. On AArch64, you can define a 2 GB contiguous slice of address space, built up out of whatever page size you find convenient for your system and plant a relocatable binary into it, up to 2 GB of size. Any instruction anywhere in the last page can reach any address in the first page, and vice versa. In RISC-V, if you try to do the same thing, you'd find that while any instructions in the last page can reach any address in the first page with room to spare. But some instructions in the first page cannot reach portions of the last page. Sure, it doesn't matter most of the time. It isn't ever really an obstacle in practice for the feller writing application code for the platform. But the linker has to be aware of it in the 'medium' code model as a special case for just this particular platform. Somebody had to write that special case to work around the hardware.
  - brucehoult 8y ago
    The linker code to calculate the necessary auipc and remaining offset for a relocation and do something else when out of bounds, was written years ago, is two lines of code, and no doubt took less time than this conversation. I don't even know of any application that has 1 GB of code, let alone 2 GB minus 2 KB (2,147,481,600 bytes).