[PATCH] D71323: [profile] Avoid allocating a page on the stack, NFC

[Vedant Kumar via Phabricator via llvm-commits]

vsk created this revision.
vsk added reviewers: arphaman, davidxl, phosek.
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When writing out a profile, avoid allocating a page on the stack for the
purpose of writing out zeroes, as some embedded environments do not have
enough stack space to accomodate this.

Instead, use a small, fixed-size zero buffer that can be written
repeatedly.

For a synthetic file with >100,000 functions, I did not measure a
significant difference in profile write times. We are removing a
page-length zero-fill `memset()` in favor of several smaller buffered
`fwrite()` calls: in practice, I am not sure there is much of a
difference. The performance impact is only expected to affect the
continuous sync mode (%c) -- zero padding is less than 8 bytes in all
other cases.

rdar://57810014


https://reviews.llvm.org/D71323

Files:
  compiler-rt/lib/profile/InstrProfilingFile.c
  compiler-rt/lib/profile/InstrProfilingInternal.h
  compiler-rt/lib/profile/InstrProfilingPlatformFuchsia.c
  compiler-rt/lib/profile/InstrProfilingWriter.c

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