Contants generation - proposal

Hi all,

I analyzed more and ready (1) to describe the problem and (2) I have a proposal.

(1) Problem description:
In X86_64 target the Code Model is "Large". It means that address is 64-bit and IP-relative memory operand can't be used in this case.
(Because in IP-relative memory operand the displacement is 32-bit).

In order to load constant, we use 2 instructions.
movabsq $.LCPI0_0, %rcx
vmulpd (%rcx), %ymm0, %ymm0

It happens because .LCPI0_0 is in .rodata section and instruction itself is in .text.

If I put the constant in .text, the code will look much better:
vmulpd .LCPI0_0(%rip), %ymm0, %ymm0

(2) Proposal
Define one more Code Model, let's say "LargeNearConst", which will allow to put constants in .text.

I'd like to hear people's opinion. Will it help other targets? I'm ready to send a patch.

Thank you.

- Elena

Hi Elena,

(2) Proposal
Define one more Code Model, let's say "LargeNearConst", which will allow to put constants in .text.

Isn't that a little heavy-handed? The large model only requires the
less efficient access for symbols we can't control, and in fact x86
still uses pc-relative conditional branches within a function so it
can't pretend to support a single function >2GB in code size even now.

I think that the improved behaviour for consts should be acceptable in
the large model. But that's just me.

I'd like to hear people's opinion. Will it help other targets? I'm ready to send a patch.

An analogous improvement would be useful for AArch64, which means that
if the new model *is* justified it would probably be useful there too.

Cheers.

Tim.

I think that the improved behavior for consts should be acceptable in the large model. But that's just me.

By default, all constants should be in a special read-only section, and this section may be far from the text section.

I'm working with JIT model or with only one object file. The code model, when all constants are near, we can call "LargeJIT". (Is it sounds better then LargeNearConst?)
For X86 target JITDefault will be translated CodeModel::LargeJIT for x64 and CodeModel::Small for 32-bit.

And the TargetLoweringObjectFile already has CodeModel inside. So the code will look like :

const MCSection *
TargetLoweringObjectFile::getSectionForConstant(SectionKind Kind) const {
  if (getCodeModel() == CodeModel::LargeJIT)
    return TextSection;
  if (Kind.isReadOnly() && ReadOnlySection != 0)
    return ReadOnlySection;

  return DataSection;
}

- Elena

I think that the improved behavior for consts should be acceptable in the large model. But that's just me.

By default, all constants should be in a special read-only section, and this section may be far from the text section.

Why should they? The only reason I can think of is to support
execute-only pages, but isn't that the less common use-case? From what
I could tell from a quick search (very non-thorough), none of the
mainstream x86 operating systems enable it by default.

Cheers.

Tim.

All compilers do this, gcc, for example
#include <stdio.h>

const char A[] = {"Hello, world"};

int main() {
  printf("%s\n", A);
  return 0;
}

gcc hello.c -o hello

objdump --all hello | grep -w A
00000000004005d8 g O .rodata 000000000000000d A

- Elena