Vedant Kumar 379d9c1dc6 [ubsan] Skip null checks on pointers to the start of an alloca ...

Pointers to the start of an alloca are non-null, so we don't need to
emit runtime null checks for them.

Testing: check-clang, check-ubsan.

This significantly reduces the amount of null checks we emit when
compiling X86ISelLowering.cpp. Here are the numbers from patched /
unpatched clangs based on r300371.

  -------------------------------------
  | Setup          | # of null checks |
  -------------------------------------
  | unpatched, -O0 |            45439 |
  | patched, -O0   |            25251 | (-44.4%)
  -------------------------------------

llvm-svn: 300508

2017-04-17 22:26:07 +00:00

174 KiB

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