assigned to @MatzeB

Some local discussion on IRC points out that https://github.com/llvm-mirror/llvm/blob/003f1a56c5e496c7c195122de24b1272d32866a0/lib/Analysis/ValueTracking.cpp#L3098-L3099 may be related here?

I believe the minimal diff for fixing this to be:

• if (GV->getInitializer()->isNullValue()) {

• if (GV->getInitializer()->isNullValue() && TrimAtNul) {

This will remove the ability to optimize certain libcalls on all-zero static buffers, but at least it won't be incorrect.
The ideal fix would probably be to allocate an all-zero string buffer if TrimAtNul is false.

Checking https://godbolt.org/g/QroaoK against various versions of clang suggests this bug has been around since LLVM 3.7, and is not present in other compilers.

I haven't stepped throught with a debugger, but you may want to confirm that SimplifyLibCall doesn't do anything hazy with memchr()

Value *LibCallSimplifier::optimizeMemChr(CallInst *CI, IRBuilder<> &B)

You may want to try this patch (at least as stopgap solution).

diff --git a/lib/Transforms/Utils/SimplifyLibCalls.cpp b/lib/Transforms/Utils/SimplifyLibCalls.cpp
index ec33679..5553d5c 100644
--- a/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ b/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -666,6 +666,10 @@ Value *LibCallSimplifier::optimizeMemChr(CallInst *CI, IRBuilder<> &B) {
// return null if we don't find the char.
Str = Str.substr(0, LenC->getZExtValue());

• // memchr("", 0, y) -> ""
• if (Str.empty() && CharC->isNullValue())
• return CI->getArgOperand(0);
• // If the char is variable but the input str and length are not we can turn
  // this memchr call into a simple bit field test. Of course this only works
  // when the return value is only checked against null.

IMO this should be fixed (even as an workaround) in getConstantStringInfo (see my mini-diff above), because memchr is unlikely to be the only affected function.

For the record: Looks like Eli found the (existing) bug while reading my review at https://reviews.llvm.org/D32839#inline-284908

The latest version of that patch fixes this issue as a side effect of some refactoring.

Fixed by r303461