[llvm-dev] [EXTERNAL] Re: [RFC] Removing optimization size level from LLVM and relying on minsize/optsize

[Snider, Todd via llvm-dev]

Hi Arthur,

If -Oz/-Os are implemented via front-end setting the optnone/minsize attribute, would that have any impact on code-size related IPOs like machine outlining?

~ Todd

From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces at lists.llvm.org>> On Behalf Of Matt Arsenault via llvm-dev
Sent: Friday, November 12, 2021 2:10 PM
To: Arthur Eubanks <aeubanks at google.com<mailto:aeubanks at google.com>>
Cc: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject: [EXTERNAL] Re: [llvm-dev] [RFC] Removing optimization size level from LLVM and relying on minsize/optsize



On Nov 12, 2021, at 15:03, Arthur Eubanks via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Currently in the LLVM IR optimization pipelines we pass around an OptimizationLevel, which consists of a speedup level and a size level (e.g. -O1 is {1, 0}, -Oz is {2, 2}). We use the size level to turn on/off some passes and also to determine inliner thresholds.

When attempting to add support for -Os/-Oz in https://reviews.llvm.org/D113738, I got some pushback saying that we should be relying on the function attributes minsize and optsize. The logical extension of that is to completely remove the size level from OptimizationLevel and rely on frontends to set minsize/optsize for -Os/-Oz. Passes that are disabled with -Os/-Oz can check those attributes instead.

There are some tests (e.g. inline-optsize.ll) that test that if we have optsize and -Oz, the lower inlining threshold (-Oz in this case) wins, but perhaps we can revisit that and calculate inline thresholds purely based on the function attributes.

Any thoughts?


I do not believe in encoding optimization levels in the IR. The optimization level is an option for the machinery of the compiler, and not part of the semantics of the program.

-Matt
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