[PATCH] D140931: Improve and enable folding of conditional branches with tail calls.

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n added a comment.

In D140931#4025075 <https://reviews.llvm.org/D140931#4025075>, @pengfei wrote:

> I'm not sure if we can arbitrarily replace `jmp foo` with `jcc foo`. At least `jmp` has a large range (64-bit) scope which supports code size > 2GB. Maybe we should limit it to kernel/small code model?

Can it? https://www.felixcloutier.com/x86/jmp I though `rel32` was maximum imm encoding.

But in `canMakeTailCallConditional` we have:

  if (TailCall.getOpcode() != X86::TCRETURNdi &&
      TailCall.getOpcode() != X86::TCRETURNdi64) {
    // Only direct calls can be done with a conditional branch.
    return false;
  }

We could limit `X86::TCRETURNdi64` with kernel/small-code model. Although this is done after
reg alloc and only within function so we could only do if function has less than 2^32 / 16 instructions
and be safe.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D140931/new/

https://reviews.llvm.org/D140931