Date: Wed, 17 Jun 2020 11:20:09 +0100 From: Alexander Richardson <arichardson@freebsd.org> To: Cy Schubert <Cy.Schubert@cschubert.com> Cc: Ian Lepore <ian@freebsd.org>, Toomas Soome <tsoome@freebsd.org>, src-committers <src-committers@freebsd.org>, svn-src-all <svn-src-all@freebsd.org>, svn-src-head <svn-src-head@freebsd.org> Subject: Re: svn commit: r362217 - head/stand/common Message-ID: <CA%2BZ_v8qjhde2cTSSaP%2BcC1aNvKUH%2BYhN3kB=dsM_15VFMwR_LA@mail.gmail.com> In-Reply-To: <202006161708.05GH8ibS019561@slippy.cwsent.com> References: <202006160705.05G753T4057972@repo.freebsd.org> <55903c38d363aef2a6f6d0075dd4526b86d51258.camel@freebsd.org> <202006161708.05GH8ibS019561@slippy.cwsent.com>
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On Tue, 16 Jun 2020 at 18:09, Cy Schubert <Cy.Schubert@cschubert.com> wrote: > > In message <55903c38d363aef2a6f6d0075dd4526b86d51258.camel@freebsd.org>, > Ian Le > pore writes: > > On Tue, 2020-06-16 at 07:05 +0000, Toomas Soome wrote: > > > Author: tsoome > > > Date: Tue Jun 16 07:05:03 2020 > > > New Revision: 362217 > > > URL: https://svnweb.freebsd.org/changeset/base/362217 > > > > > > Log: > > > loader: variable i is unused without MBR/GPT support built in > > > > > > Because i is only used as index in for loop, declare it in for > > > statement. > > > > > > > As much as I prefer doing it this way, style(9) doesn't allow for > > variable declarations inside a for() statement (or even inside a local > > block, which is just too 1980s for me, but it is still our standard). > > Doesn't this use stack for a shorter period of time or does the compiler > optimize this, making this change moot? > > The tradeoff is a few extra bytes of stack for a longer period of time vs a > few extra instructions incrementing and decrementing the stack pointer. > > At least for LLVM it makes no difference where you declare the variable: it will always be represented by an alloca instruction at the start of the IR function: https://godbolt.org/z/NFG3hN There will also not be any changes to the stack pointer since the stack frame size is fixed no matter where the variables are declared (unless you use variable-length arrays or __builtin_alloca()). LLVM will also merge variables that have non-overlapping lifetimes to reuse the same stack slot if possible. I'm almost certain GCC performs the same optimizations (as will any other compiler written in the last 20 years). Alexhome | help
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