Re: [PATCH], Add configuration checks to PowerPC --with-long-double-format=ieee

[Joseph Myers <joseph at codesourcery dot com>]

On Fri, 6 Jul 2018, Segher Boessenkool wrote:

> Version checks are terrible.  This is nothing new.

The key principle behind --with-glibc-version is that you can pass that 
option *when building the static-only inhibit_libc bootstrap compiler 
without having built glibc yet* and it will result in the compiler being 
correctly configured for the specified glibc version, and thus able to 
build glibc binaries identical to those you get from a longer alternating 
sequence of compiler and glibc (headers) builds.

At that point in a bootstrap of a cross toolchain you don't have any 
target glibc headers available (you might have target kernel headers) and 
so have no other way in which the compiler can possibly tell what glibc 
version is in use.

> For cross builds you can just assume it works.  That should work fine here.

We definitely support building a new GCC using a sysroot of an old glibc 
(so that the new GCC can then be used to build binaries that will run on 
old distributions, for example).  (This certainly works and is useful for 
x86_64; I don't assert whether or not it works, or makes sense, for 
powerpc64le.)

Of course in that case the build can examine target headers to determine 
versions and features, but a new GCC release is still likely to be able to 
build a few past glibc release branches if anyone backported the required 
build fixes to those branches, and so the bootstrap case is still useful 
with somewhat older glibc versions.

-- 
Joseph S. Myers
joseph@codesourcery.com