[AggressiveInstCombine] Inline strcmp/strncmp (#89371)

Inline calls to strcmp(s1, s2) and strncmp(s1, s2, N), where N and
exactly one of s1 and s2 are constant.

For example:

```c
int res = strcmp(s, "ab");
```

is converted to

```c
int res = (int)s[0] - (int)'a';
if (res != 0)
  goto END;
res = (int)s[1] - (int)'b';
if (res != 0)
  goto END;
res = (int)s[2] - (int)'\0';
END:
```

Ported from a similar gcc feature [Inline strcmp with small constant
strings](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78809).

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index fed2061..0dbb39d 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -252,6 +252,13 @@ bool llvm::haveNoCommonBitsSet(const WithCache<const Value *> &LHSCache,
                                         RHSCache.getKnownBits(SQ));
 }
 
+bool llvm::isOnlyUsedInZeroComparison(const Instruction *I) {
+  return !I->user_empty() && all_of(I->users(), [](const User *U) {
+    ICmpInst::Predicate P;
+    return match(U, m_ICmp(P, m_Value(), m_Zero()));
+  });
+}
+
 bool llvm::isOnlyUsedInZeroEqualityComparison(const Instruction *I) {
   return !I->user_empty() && all_of(I->users(), [](const User *U) {
     ICmpInst::Predicate P;