[InferAttrs] Add writeonly to all the math functions

All of these functions would be `readnone`, but can't be on platforms
where they can set `errno`. A `writeonly` function with no pointer
arguments can only write (but never read) global state.

Writeonly theoretically allows these calls to be CSE'd (a writeonly call
with the same arguments will always result in the same global stores) or
hoisted out of loops, but that's not implemented currently.

There are a few functions in this list that could be `readnone` instead
of `writeonly`, if someone is interested.

Differential Revision: https://reviews.llvm.org/D116426

llvm/lib/Transforms/Utils/BuildLibCalls.cpp

@@ -34,6 +34,7 @@ STATISTIC(NumReadNone, "Number of functions inferred as readnone");
 STATISTIC(NumInaccessibleMemOnly,
           "Number of functions inferred as inaccessiblememonly");
 STATISTIC(NumReadOnly, "Number of functions inferred as readonly");
+STATISTIC(NumWriteOnly, "Number of functions inferred as writeonly");
 STATISTIC(NumArgMemOnly, "Number of functions inferred as argmemonly");
 STATISTIC(NumInaccessibleMemOrArgMemOnly,
           "Number of functions inferred as inaccessiblemem_or_argmemonly");
@@ -71,6 +72,14 @@ static bool setOnlyReadsMemory(Function &F) {
   return true;
 }
 
+static bool setDoesNotReadMemory(Function &F) {
+  if (F.doesNotReadMemory()) // writeonly or readnone
+    return false;
+  F.setDoesNotReadMemory();
+  ++NumWriteOnly;
+  return true;
+}
+
 static bool setOnlyAccessesArgMemory(Function &F) {
   if (F.onlyAccessesArgMemory())
     return false;
@@ -1171,6 +1180,7 @@ bool llvm::inferLibFuncAttributes(Function &F, const TargetLibraryInfo &TLI) {
   case LibFunc_truncl:
     Changed |= setDoesNotThrow(F);
     Changed |= setDoesNotFreeMemory(F);
+    Changed |= setDoesNotReadMemory(F);
     Changed |= setWillReturn(F);
     return Changed;
   default: