Walking Around the Compiler

Posted3 months agoActive3 months ago

chunkles

52 points

11 comments

bernsteinbear.comTechstory

calmmixed

Debate

60/100

Compiler OptimizationProgramming LanguagesPypy

Key topics

Compiler Optimization

Programming Languages

Pypy

The article discusses how the PyPy team encountered an issue with the compiler optimizer and how they worked around it, sparking a discussion on the fix and its potential implications.

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60-66h

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01Story posted
Sep 23, 2025 at 7:08 PM EDT
3 months ago
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Sep 26, 2025 at 6:13 AM EDT
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9 comments in 60-66h
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Sep 26, 2025 at 10:59 AM EDT
3 months ago
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Discussion (11 comments)

Showing 11 comments

zabzonk

3 months ago

1 reply

> surely the optimizer can reason that the float_abs operation produces a positive number!

how?

pm215

3 months ago

1 reply

By having knowledge baked into it about its properties (which it can validly do because the behaviour of the float_abs operation is specified by the language; it's not calling an arbitrary external function). The blog post sketches out one approach to this: as the optimizer is working on a tree of expressions, it has attached to the nodes extra information about the values in it (e.g. "this is a constant X" or "this value is definitely in the range A..B"); then as it simplifies the tree, it can say "abs of a value that's already definitely not negative is a no-op, don't do anything", in the same way it can say "multiplying X by 1 is a no-op".

The actual bug-fix linked to in the footnote does it in a slightly different way: it says "as I'm walking through optimizing my tree of expressions, if I see 'abs(abs(X))' then simplify the tree to just 'abs(X)'".

zabzonk

3 months ago

1 reply

> the behaviour of the float_abs operation is specified by the language

Which language? Where? I just googled for float_abs and got nothing.

pm215

3 months ago

PyPy intermediate representation. I assume this is what PyPy turns Python "abs(x)" into in codepaths where it knows x is a float.

mtklein

3 months ago

2 replies

I don't understand why this article invents and explains a phony ranged-float fix when the real fix from the footnotes would have been just as simple to explain. The deception needlessly undermines the main point of the article, which I completely agree with.

tekknolagi

3 months ago

The real fix felt more complicated when I drafted this. Seems like it isn't; I'll think about updating the post

stassats

3 months ago

That fix has limited applicability. x * x is also a non-negative float. But abs(x * x) is not optimized. Or abs(abs(x)+1). GCC, for example, does know that.

Someone

3 months ago

2 replies

I know about nothing about PyPy internals but

  case float_abs(_):
  -          return float
  +          return float.with_range(low=0, high=None)

to me, looks like a risky change. I would fear this could introduce a bug when computing

  isNaN(abs(NaN))

That should return true, but with that change, I fear it could return false because it informs the optimizer that abs never returns a NaN.

stassats

3 months ago

That shouldn't mean that it doesn't return a NaN. Things are generally not optimized away because of NaNs. E.g. in GCC, abs(c) > -1.0 is not folded, unless building with -ffast-math

oasisaimlessly

3 months ago

Where'd you get that patch from? I can't find it in the blog post.

EDIT: Ah, other comment mentions article was edited.

etyp

3 months ago

Random note since Godbolt was mentioned: It's also fun to hop on play.rust-lang.org and see what different IRs look like via the "..." next to "RUN." Just look at how simple the HIR is pretty simple for "Hello world" - then check out the MIR ;)

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ID: 45353986Type: storyLast synced: 11/20/2025, 3:38:03 PM

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