test: make ValueTest/objects float check robust against x87 excess precision#1700
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…ecision ValueTest/objects stores the float literal 0.12345f into a Json::Value (widened to the stored double) and then asserts equality against the original 0.12345f. On 32-bit x86 targets where GCC defaults to x87 math (-mfpmath=387, 80-bit excess precision) and the baseline lacks SSE2, the round-trip yields the exact double 0.12345 rather than the float-widened 0.12345000356435776, so the exact double comparison fails even though the library itself is correct. Compare the stored value narrowed back to float via asFloat(): both the expected literal and asFloat() collapse to the same float value on every architecture, keeping the numeric round-trip check while making it precision-robust.
plusky
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Jun 25, 2026
Document the i586-only exact-float-compare test failure pattern: x87 80-bit excess precision; -fexcess-precision=standard is a no-op; -mfpmath=sse/-msse2 is unsafe (no SSE2 in the i586 baseline -> SIGILL); fix by narrowing both sides to float (asFloat()/cast); and verify via the server-side i586 build (not flaky local QEMU) before the Factory SR. Real case: jsoncpp 1.9.8 ValueTest/objects, upstream PR gh#open-source-parsers/jsoncpp#1700.
bmwiedemann
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Jun 27, 2026
https://build.opensuse.org/request/show/1361767 by user pluskalm + dimstar_suse jsoncpp 1.9.8: fix the i586 FTBFS (declined sr#1361562). ValueTest/objects did an exact float-vs-double compare that x87 80-bit excess precision broke on i586 (no SSE2 in the baseline, so -mfpmath=sse is unsafe). jsoncpp-i586-float-precision.patch compares the value narrowed back to float, robust on all arches; verified i586 build now succeeds (unittest OK, Fail: 0). Submitted upstream as gh#open-source-parsers/jsoncpp#1700.
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On 32-bit x86 (i586/i686) where GCC defaults to x87 math (
-mfpmath=387, 80-bit excess precision) and the baseline has no SSE2,ValueTest/objectsfails:The test stores the float literal
0.12345finto aJson::Value(widened to the storeddouble) and then asserts equality against the original0.12345f. On x87 the stored value round-trips to the exact double0.12345instead of the float-widened0.12345000356435776, so the exactdoublecomparison fails — even though the library is correct.This makes the check precision-robust by comparing the stored value narrowed back to
floatviaasFloat(): both sides collapse to the samefloaton every architecture, so the numeric round-trip is still verified but the test no longer depends on x87 vs IEEE double representation.Found while building jsoncpp 1.9.8 for openSUSE on the i586 architecture (no SSE2 in the baseline, so
-mfpmath=sseis not an option).