[SelectionDAG] Allow the user to specify a memeq function.

Summary:
Right now, when we encounter a string equality check,
e.g. `if (memcmp(a, b, s) == 0)`, we try to expand to a comparison if `s` is a
small compile-time constant, and fall back on calling `memcmp()` else.

This is sub-optimal because memcmp has to compute much more than
equality.

This patch replaces `memcmp(a, b, s) == 0` by `bcmp(a, b, s) == 0` on platforms
that support `bcmp`.

`bcmp` can be made much more efficient than `memcmp` because equality
compare is trivially parallel while lexicographic ordering has a chain
dependency.

Subscribers: fedor.sergeev, jyknight, ckennelly, gchatelet, llvm-commits

Differential Revision: https://reviews.llvm.org/D56593

llvm-svn: 355672

1 files changed, 13 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/TargetLibraryInfo.cpp b/llvm/lib/Analysis/TargetLibraryInfo.cpp
index 29e460ec785..50a05a897e9 100644
--- a/llvm/lib/Analysis/TargetLibraryInfo.cpp
+++ b/llvm/lib/Analysis/TargetLibraryInfo.cpp
@@ -49,6 +49,16 @@ static bool hasSinCosPiStret(const Triple &T) {
   return true;
 }
 
+static bool hasBcmp(const Triple &TT) {
+  // Posix removed support from bcmp() in 2001, but the glibc and several
+  // implementations of the libc still have it.
+  if (TT.isOSLinux())
+    return TT.isGNUEnvironment() || TT.isMusl();
+  // Both NetBSD and OpenBSD are planning to remove the function. Windows does
+  // not have it.
+  return TT.isOSFreeBSD() || TT.isOSSolaris() || TT.isOSDarwin();
+}
+
 /// Initialize the set of available library functions based on the specified
 /// target triple. This should be carefully written so that a missing target
 /// triple gets a sane set of defaults.
@@ -141,6 +151,9 @@ static void initialize(TargetLibraryInfoImpl &TLI, const Triple &T,
     TLI.setUnavailable(LibFunc_sincospif_stret);
   }
 
+  if (!hasBcmp(T))
+    TLI.setUnavailable(LibFunc_bcmp);
+
   if (T.isMacOSX() && T.getArch() == Triple::x86 &&
       !T.isMacOSXVersionLT(10, 7)) {
     // x86-32 OSX has a scheme where fwrite and fputs (and some other functions