These are some of the warnings you get when turning -Wconversion on and -Werror off.

Because of multiple files that include these, the number of warnings in the result file for some of the header files is high for only a couple of errors.

555 src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceIdMacros.hpp
535 src/hotspot/share/jfr/writers/jfrEncoders.hpp
18 src/hotspot/share/jfr/utilities/jfrBigEndian.hpp
18 src/hotspot/share/jfr/instrumentation/jfrEventClassTransformer.cpp
14 src/hotspot/share/jfr/support/jfrAdaptiveSampler.cpp
8 src/hotspot/share/jfr/periodic/jfrThreadCPULoadEvent.cpp
6 src/hotspot/share/jfr/periodic/jfrPeriodic.cpp
4 src/hotspot/share/jfr/recorder/stacktrace/jfrStackTrace.cpp
1 src/hotspot/share/jfr/recorder/service/jfrEventThrottler.cpp
1 src/hotspot/share/jfr/recorder/repository/jfrChunkWriter.cpp
1 src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceIdKlassQueue.cpp
1 src/hotspot/share/jfr/recorder/checkpoint/types/jfrThreadGroup.cpp

src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceIdMacros.hpp:67:83: warning: conversion from 'traceid' {aka 'long unsigned int'} to 'jbyte' {aka 'signed char'} may change value [-Wconversion]        
   67 | #define THIS_EPOCH_BIT                            (JfrTraceIdEpoch::this_epoch_bit())
      |                                                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceIdMacros.hpp:81:74: note: in definition of macro 'TRACE_ID_TAG'
   81 | #define TRACE_ID_TAG(ptr, bits)                   (JfrTraceIdBits::store(bits, ptr))
      |                                                                          ^~~~
src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceIdMacros.hpp:111:70: note: in expansion of macro 'THIS_EPOCH_BIT'
  111 | #define SET_USED_THIS_EPOCH(ptr)                  (TRACE_ID_TAG(ptr, THIS_EPOCH_BIT))
      |                                                                      ^~~~~~~~~~~~~~
src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceIdLoadBarrier.inline.hpp:70:5: note: in expansion of macro 'SET_USED_THIS_EPOCH'
   70 |     SET_USED_THIS_EPOCH(klass);
      |     ^~~~~~~~~~~~~~~~~~~
src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceIdLoadBarrier.inline.hpp: In static member function 'static traceid JfrTraceIdLoadBarrier::load(const Klass*, const Method*)':
src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceIdMacros.hpp:71:101: warning: conversion from 'traceid' {aka 'long unsigned int'} to 'jbyte' {aka 'signed char'} may change value [-Wconversion]       
   71 | #define THIS_EPOCH_METHOD_AND_CLASS_BITS          (JfrTraceIdEpoch::this_epoch_method_and_class_bits())
      |                                                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceIdMacros.hpp:81:74: note: in definition of macro 'TRACE_ID_TAG'
   81 | #define TRACE_ID_TAG(ptr, bits)                   (JfrTraceIdBits::store(bits, ptr))
      |                                                                          ^~~~
src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceIdMacros.hpp:112:70: note: in expansion of macro 'THIS_EPOCH_METHOD_AND_CLASS_BITS'
  112 | #define SET_METHOD_AND_CLASS_USED_THIS_EPOCH(kls) (TRACE_ID_TAG(kls, THIS_EPOCH_METHOD_AND_CLASS_BITS))
      |                                                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceIdLoadBarrier.inline.hpp:92:6: note: in expansion of macro 'SET_METHOD_AND_CLASS_USED_THIS_EPOCH'
   92 |      SET_METHOD_AND_CLASS_USED_THIS_EPOCH(klass);
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~