[CIR] Change default assumption about allowing builtins (#144004)

The code to read the "nobuiltins" attributes hasn't been implemented
yet, but we were defaulting to the assumption that use of builtins is
allowed for function calls that we recognize as standard C library calls
and have builtin equivalents of. This change reverses that assumption so
that when such calls are encountered, we just emit the call. This is a
better default assumption, and since our builtin handling for these
functions isn't implemented yet, it also allows us to compile more
programs.

Author: andykaylor

clang/lib/CIR/CodeGen/CIRGenExpr.cpp

@@ -1056,7 +1056,8 @@ CIRGenCallee CIRGenFunction::emitDirectCallee(const GlobalDecl &gd) {
 
     bool isPredefinedLibFunction =
         cgm.getASTContext().BuiltinInfo.isPredefinedLibFunction(builtinID);
-    bool hasAttributeNoBuiltin = false;
+    // Assume nobuiltins everywhere until we actually read the attributes.
+    bool hasAttributeNoBuiltin = true;
     assert(!cir::MissingFeatures::attributeNoBuiltin());
 
     // When directing calling an inline builtin, call it through it's mangled

clang/test/CIR/CodeGen/libc.c

@@ -0,0 +1,55 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s -o %t.cir
+// RUN: FileCheck --input-file=%t.cir %s
+
+// Note: In the final implementation, we will want these to generate
+//       CIR-specific libc operations. This test is just a placeholder
+//       to make sure we can compile these to normal function calls
+//       until the special handling is implemented.
+
+void *memcpy(void *, const void *, unsigned long);
+void testMemcpy(void *dst, const void *src, unsigned long size) {
+  memcpy(dst, src, size);
+  // CHECK: cir.call @memcpy
+}
+
+void *memmove(void *, const void *, unsigned long);
+void testMemmove(void *src, const void *dst, unsigned long size) {
+  memmove(dst, src, size);
+  // CHECK: cir.call @memmove
+}
+
+void *memset(void *, int, unsigned long);
+void testMemset(void *dst, int val, unsigned long size) {
+  memset(dst, val, size);
+  // CHECK: cir.call @memset
+}
+
+double fabs(double);
+double testFabs(double x) {
+  return fabs(x);
+  // CHECK: cir.call @fabs
+}
+
+float fabsf(float);
+float testFabsf(float x) {
+  return fabsf(x);
+  // CHECK: cir.call @fabsf
+}
+
+int abs(int);
+int testAbs(int x) {
+  return abs(x);
+  // CHECK: cir.call @abs
+}
+
+long labs(long);
+long testLabs(long x) {
+  return labs(x);
+  // CHECK: cir.call @labs
+}
+
+long long llabs(long long);
+long long testLlabs(long long x) {
+  return llabs(x);
+  // CHECK: cir.call @llabs
+}