build: give a cross build's mrbc the target's answer on floats - #7230

A MRuby::CrossBuild that names no mrbc of its own takes one from the host
build, and where the build config declares none, CrossBuild#initialize
generates a minimal one (lib/mruby/build.rb:618-625 on master): a bare
toolchain + build_mrbc_exec + disable_libmruby, carrying none of the
target's configuration.

mrbc and the target therefore need not agree about what a value is, and
src/load.c does not let that pass. A pool entry the target cannot represent
makes it refuse the whole irep:

      case IREP_TT_FLOAT:
#ifndef MRB_NO_FLOAT
        if (src + sizeof(double) > end) return FALSE;
        pool[i].tt = tt;
        pool[i].u.f = str_to_double(mrb, (const char*)src);
        src += sizeof(double);
        break;
#else
        return FALSE;           /* MRB_NO_FLOAT */
#endif

So one float literal, however far it sits from the code being run, costs the
file it is written in:

$ cat t.rb
p 1.5
$ build/host/bin/mrbc -o t.mrb t.rb        # the generated host mrbc, floats enabled
$ build/no-float/bin/mruby -b t.mrb        # the target, MRB_NO_FLOAT
(unknown):0: irep load error (ScriptError)

build_config/no-float.rb is such a build. Compiling each of test/t/*.rb with
its mrbc and loading the result on its own bin/mruby leaves twelve files
unloadable (array, class, float, gc, hash, integer, kernel,
literals, numeric, range, string, vformat), and rake test ends at
the first of them:

$ rake test MRUBY_CONFIG=build_config/no-float.rb
...
TEST for no-float
mrbtest - Embeddable Ruby Test

.........trace (most recent call last):
test/t/argumenterror.rb:34: irep load error (ScriptError)
rake aborted!

The name in that trace is the frame that was running, not the file that failed
to load; the failure carries no information about where the literal is.

Fix

Bind each cross target to an mrbc that answers the float question the way it
does, and let targets that answer alike share one:

• a host that agrees is borrowed, as before;
• where there is no host, the generated build takes that name, as before, and
  carries the target's answer;
• a host the build config declares itself is left as it is written, so a
  target that disagrees with it gets a private mrbc beside its own output, at
  <target>/mrbc, the way a native build gets one from create_mrbc_build.

mrbc then refuses a float literal where it is written, naming the file and the
line, instead of emitting bytecode that fails at load:

$ rake test MRUBY_CONFIG=build_config/no-float.rb
...
test/t/array.rb:65: Not implemented: PM_FLOAT_NODE
test/t/array.rb:0:0: generator error, Not implemented: PM_FLOAT_NODE

which is what build_config/host-nofloat.rb has always done, its mrbc being
its own because it is not a cross build. The remaining distance to a suite that
runs is the float literals in the shared test files, which this does not touch;
it only moves the report to the line that causes it.

Command::Compiler#has_define? is the reader that answers here: MRB_NO_FLOAT
is a build-config define, and Build#has_define? refuses to answer this early
because the gems have not contributed theirs yet.

Scope

Only MRB_NO_FLOAT, and only where the target and the mrbc it would borrow
disagree about it. A cross build that names its own mrbc (conf.mrbcfile =)
is untouched, as is one that agrees with the host it borrows from.

Verification

Two cross builds in one config, one MRB_NO_FLOAT and one not, built in both
declaration orders. Each target's mrbc compiles p 1.5, and the result is
loaded on that target's own bin/mruby:

The two orders agree, and neither target is served an mrbc that disagrees
with it.

A build config that declares its own float-enabled host next to an
MRB_NO_FLOAT cross build keeps that host and gets a separate mrbc for the
cross target:

float OK    host/bin/mrbc
float OK    host/mrbc/bin/mrbc      # the native build's own, from create_mrbc_build
float DENY  nf/mrbc/bin/mrbc

rake test, build_config/ci/gcc-clang.rb, all five builds plus bintest:

No CI config declares a MRuby::CrossBuild (ci/gcc-clang, ci/msvc and
cosmopolitan are all MRuby::Build), so nothing there reaches this code.

The path that does is checked directly, with build_config/no-float.rb minus
the define, so its suite runs natively and the mrbc selection is the only
thing under test:

MRuby::CrossBuild.new('cross-float') do |conf|
  conf.toolchain
  conf.gem :core => "mruby-bin-mruby"
  conf.test_runner.command = 'env'
  conf.enable_debug
  conf.enable_test
end

And build_config/no-float.rb itself, which builds and runs as before:

$ rake MRUBY_CONFIG=build_config/no-float.rb          # exit 0
$ build/no-float/bin/mruby -e 'p :sym, 1+2, "s".upcase, [1,2].map{|x|x*2}'
:sym
3
"S"
[2, 4]

Environment

No C source changes, so .text is unaffected in every build.

🤖 Generated with Claude Code

https://claude.ai/code/session_01DRc6K8kwzLzvs4W6GiLRaz

Summary by CodeRabbit

• Bug Fixes
  • Improved cross-compilation reliability when builds use different floating-point configurations.
  • Automatically selects a compatible build tool or creates one with matching settings, preventing configuration conflicts.
  • Ensures generated tools use the correct target configuration for more consistent build results.