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Re: __builtin_assume warnings

From: Paul Eggert
Subject: Re: __builtin_assume warnings
Date: Tue, 18 Aug 2020 15:53:54 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 8/18/20 12:11 AM, Mattias Engdegård wrote:

Wouldn't -Wno-assume, either locally (clang pragmas around __builtin_assume) or 
globally (configure?) be more effective?

No, it's the other way around at least for me: having 'assume' use Clang's __builtin_assume makes 'assume' slower. Without __builtin_assume, 'assume' falls back on __builtin_unreachable, and Clang generates better code for __builtin_unreachable than it does __builtin_assume. For the following code:

int x;
static int f (void) { return x; }
int g (void) { __builtin_assume (!f ()); return f (); }
int h (void) { if (f ()) __builtin_unreachable (); return f (); }

clang -O2 generates suboptimal machine code for g (the generated code loads from 'x') and better machine code for h (the generated code returns 0 without loading from 'x'). This is clang version 9.0.1 (Fedora 9.0.1-2.fc31) on x86_64-unknown-linux-gnu.

Perhaps someday the Clang folks will get their act together in this department, but in the meantime __builtin_unreachable is a perfectly adequate substitute.

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