ruby-dev and Ruby < 3.2 compiled with -O0 on Fedora 42+ #2637

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1. eregon

   added

   bug

   on Aug 31, 2026
2. eregon commented on Aug 31, 2026

   

   eregon

   on Aug 31, 2026

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   Using cflags instead of CFLAGS is the obvious fix, that appends instead of overrides, giving:

   $ ruby -e 'p RbConfig::CONFIG["CFLAGS"]'                            
   "-fstack-protector-strong -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -std=gnu17  -O3 -fno-fast-math -ggdb3 -Wall -Wextra -Wdeprecated-declarations -Wdiv-by-zero -Wduplicated-cond -Wimplicit-function-declaration -Wimplicit-int -Wpointer-arith -Wwrite-strings -Wold-style-definition -Wimplicit-fallthrough=0 -Wmissing-noreturn -Wno-cast-function-type -Wno-long-long -Wno-missing-field-initializers -Wno-overlength-strings -Wno-packed-bitfield-compat -Wno-tautological-compare -Wno-unused-parameter -Wno-unused-value -Wsuggest-attribute=format -Wsuggest-attribute=noreturn -Wunused-variable -Wmisleading-indentation -Wundef  -fPIC"
   $ ruby -rbenchmark -e 'p Benchmark.realtime { 30_000_000.times {} }'
   0.8652950529976806
   

   diff --git i/bin/ruby-build w/bin/ruby-build
   index 52fc7e0b..c5fb8b36 100755
   --- i/bin/ruby-build
   +++ w/bin/ruby-build
   @@ -748,7 +748,15 @@ build_package_standard() {
          #
          # TODO: Consider changing this to check for GCC v15 specifically
          # rather than inspecting the OS itself.
   -      export CFLAGS="-std=gnu17 $CFLAGS"
   +      if [ "${CFLAGS+defined}" ]; then
   +        export CFLAGS="-std=gnu17 $CFLAGS"
   +      else
   +        # Use the `cflags` configure variable, which Ruby's configure appends to
   +        # its default optflags/debugflags/warnflags, unlike CFLAGS which would
   +        # override them and notably lose the -O3, producing a much slower Ruby.
   +        # configure ignores `cflags` when CFLAGS is given, hence the branch above.
   +        export cflags="-std=gnu17 $cflags"
   +      fi
        fi
        # ./configure --prefix=/path/to/ruby
        # shellcheck disable=SC2086,SC2153
   

   There is also another bug here, that ruby-dev is considered < 3.2.
3. eregon commented on Aug 31, 2026

   

   eregon

   on Aug 31, 2026

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   BTW RUBY_CFLAGS also overrides CFLAGS, which means getting -O0 unless one has a -O flag in RUBY_CFLAGS.
4. mislav

   closed this as completedin #2638on Sep 2, 2026
5. eregon

   added a commit that references this issue on Oct 23, 2026

   fix(rpm-armhf): pass the ARM architecture as a compile flag

   ...

   efad824