In our development builds we log mangled memory with:
  log_develop_trace(gc)("Mangling [" PTR_FORMAT " to " PTR_FORMAT ")", p2i(mr.start()), p2i(mr.end()));
  Copy::fill_to_words(mr.start(), mr.word_size(), badHeapWord);
  log_develop_trace(gc)("Mangling done.");

This leads to a lot of print outs like:
[6.469s][trace][gc ] GC(26) Mangling [0x00000000fba00000 to 0x00000000fbb00000)
[6.469s][trace][gc ] GC(26) Mangling [0x00000000faa00000 to 0x00000000fab00000)
[6.470s][trace][gc ] GC(26) Mangling done.
[6.470s][trace][gc ] GC(26) Mangling done.
[6.470s][trace][gc ] GC(26) Mangling done.
[6.470s][trace][gc ] GC(26) Mangling [0x00000000fb500000 to 0x00000000fb600000)
[6.470s][trace][gc ] GC(26) Mangling done.
[6.471s][trace][gc ] GC(26) Mangling done.
[6.470s][trace][gc ] GC(26) Mangling done.
[6.470s][trace][gc ] GC(26) Mangling [0x00000000fd600000 to 0x00000000fd700000)
[6.471s][trace][gc ] GC(26) Mangling [0x00000000fbd00000 to 0x00000000fbe00000)
[6.470s][trace][gc ] GC(26) Mangling done.
[6.470s][trace][gc ] GC(26) Mangling [0x00000000fd200000 to 0x00000000fd300000)
[6.471s][trace][gc ] GC(26) Mangling [0x00000000fbf00000 to 0x00000000fc000000)
[6.471s][trace][gc ] GC(26) Mangling done.
[6.470s][trace][gc ] GC(26) Mangling [0x00000000fb300000 to 0x00000000fb400000)
[6.471s][trace][gc ] GC(26) Mangling [0x00000000fde00000 to 0x00000000fdf00000)
[6.470s][trace][gc ] GC(26) Mangling done.
...

I propose that we remove this noisy output for now.

It might be worth having a zap logging tag, but then it would be good to add it to all other places where we zap memory.