[all-commits] [llvm/llvm-project] 3a61e7: [ARM][Thumb1] Improve EstimateFunctionSizeInBytes ...

[Simon Tatham via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 3a61e727bc056024bdf4960e914ec6651fd41a72
      https://github.com/llvm/llvm-project/commit/3a61e727bc056024bdf4960e914ec6651fd41a72
  Author: Simon Tatham <simon.tatham at arm.com>
  Date:   2026-07-13 (Mon, 13 Jul 2026)

  Changed paths:
    M llvm/lib/Target/ARM/ARMFrameLowering.cpp
    M llvm/lib/Target/ARM/ARMMachineFunctionInfo.h
    M llvm/test/CodeGen/ARM/estimate-size-copy.mir

  Log Message:
  -----------
  [ARM][Thumb1] Improve EstimateFunctionSizeInBytes accuracy (#203318)

The estimate of a function size now includes (what I hope are) upper
bounds on the size of the prologue and epilogue; adds size estimates for
some pseudo-instructions that were previously counted as 0; and
increases the estimates for things that were previously optimistic, such
as assuming no constant pool entry ever needs to be duplicated. The
estimation function is also passed extra information to use for
adjusting its estimates, such as the BigFrameOffsets flag which makes
some pseudos have much longer expansions.

Background:

EstimateFunctionSizeInBytes is supposed to estimate how large a Thumb1
function will end up, in advance of actually doing the full code
generation. It must overestimate rather than underestimating, because
large functions need a special precaution (namely, making sure LR is
stacked, so that BL can be used for an intra-function long branch). In
multiple cases recently it has underestimated, leading to a crash later
in code generation, when ARMConstantIslandsPass needs to insert an
intra-function BL and finds that it isn't safe to corrupt LR.

Discussion on Discourse suggested that it's OK to overestimate by quite
a large factor, because any significantly large function won't be badly
impacted by a push of LR that turns out to be unnecessary. The main
place where we want to leave out those unnecessary pushes is _really_
small leaf functions, where the extra push and pop might be a
significant fraction of the whole call.



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