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Calculate a score for fields

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The score is the access cost on Cortex-M0+ thumb builds, expressed in
instructions. This commit does not have any provisions to calculate
different scores based on the target architecture, so it's only meaningful
when running this script with `-t armv6m-unknown-eabi`.

Go with a simple model: direct access is possible for the first 128 fields
of a structure, direct access costs 1 instruction, indirect access costs 3
instructions.

The score calculation assumes that each lexical use in the source code
corresponds to one access. This does not take into account unused code,
inlining, uses from application code, etc.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2021-09-16 20:04:49 +02:00
parent 53a581fe0a
commit 766ff6802e
1 changed files with 18 additions and 3 deletions
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21
scripts/field_report.py
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@ -52,6 +52,21 @@ class FieldInfo:
def uses(self) -> int:
return self.lexical_uses
def is_indirect(self) -> bool:
"""Whether access to this field is indirect on ARM Cortex-M0+."""
word_offset = self.offset // self.align
return word_offset >= 128
def score(self) -> int:
"""A field's score is an estimate of its access cost.
The cost is calculated as the number of instructions needed to
access the field on ARM Cortex-M0+, multiplied by the number of uses
of the field inside the library.
"""
cost_per_use = 1 + 2 * self.is_indirect()
return self.uses() * cost_per_use
class Ast:
"""Abstract representation of the source code."""
@ -166,12 +181,12 @@ class Ast:
prefix: str, field: FieldInfo) -> None:
"""Print information about a structure field.
Format: <type>.<name>,<size>,<alignment>,<offset>,use_count
Format: <type>.<name>,<size>,<alignment>,<offset>,<use_count>,<score>
"""
out.write('{},{},{},{},{}\n'.format(
out.write('{},{},{},{},{},{}\n'.format(
prefix + field.node.spelling,
field.size, field.align, field.offset,
field.uses()
field.uses(), field.score()
))
def report_fields(self, out: typing_util.Writable) -> None: