[FastISel] Don't force SDAG fallback for libcalls (#171782)

The fast instruction selector should should not force an SDAG fallback
to potentially make use of optimized libcall implementations.

Looking at
3e6fa46,
part of the motivation was to avoid libcalls in unoptimized builds for
targets that don't have them, but I believe this should be handled by
Clang directly emitting intrinsics instead of libcalls (which it already
does). FastISel should not second guess this.

Followup to #171288.

Author: nikic

llvm/lib/CodeGen/SelectionDAG/FastISel.cpp

@@ -1565,14 +1565,6 @@ bool FastISel::selectInstruction(const Instruction *I) {
 
   if (const auto *Call = dyn_cast<CallInst>(I)) {
     const Function *F = Call->getCalledFunction();
-    LibFunc Func;
-
-    // As a special case, don't handle calls to builtin library functions that
-    // may be translated directly to target instructions.
-    if (F && !F->hasLocalLinkage() && F->hasName() &&
-        LibInfo->getLibFunc(F->getName(), Func) &&
-        LibInfo->hasOptimizedCodeGen(Func))
-      return false;
 
     // Don't handle Intrinsic::trap if a trap function is specified.
     if (F && F->getIntrinsicID() == Intrinsic::trap &&

llvm/test/CodeGen/X86/stack-protector-msvc-oz.ll

@@ -63,11 +63,10 @@ define void @test(ptr %a) nounwind ssp minsize {
 ; MSVC-X86-O0-NEXT:    movl ___security_cookie, %eax
 ; MSVC-X86-O0-NEXT:    xorl %esp, %eax
 ; MSVC-X86-O0-NEXT:    movl %eax, {{[0-9]+}}(%esp)
-; MSVC-X86-O0-NEXT:    movl {{[0-9]+}}(%esp), %ecx
-; MSVC-X86-O0-NEXT:    movl %esp, %eax
-; MSVC-X86-O0-NEXT:    movl %ecx, 4(%eax)
+; MSVC-X86-O0-NEXT:    movl {{[0-9]+}}(%esp), %eax
 ; MSVC-X86-O0-NEXT:    leal {{[0-9]+}}(%esp), %ecx
-; MSVC-X86-O0-NEXT:    movl %ecx, (%eax)
+; MSVC-X86-O0-NEXT:    movl %ecx, (%esp)
+; MSVC-X86-O0-NEXT:    movl %eax, {{[0-9]+}}(%esp)
 ; MSVC-X86-O0-NEXT:    calll _strcpy
 ; MSVC-X86-O0-NEXT:    leal LC, %ecx
 ; MSVC-X86-O0-NEXT:    leal {{[0-9]+}}(%esp), %eax

llvm/test/CodeGen/X86/stack-protector-msvc.ll

@@ -75,11 +75,10 @@ define void @test(ptr %a) nounwind ssp {
 ; MSVC-X86-O0-NEXT:    movl ___security_cookie, %eax
 ; MSVC-X86-O0-NEXT:    xorl %esp, %eax
 ; MSVC-X86-O0-NEXT:    movl %eax, {{[0-9]+}}(%esp)
-; MSVC-X86-O0-NEXT:    movl {{[0-9]+}}(%esp), %ecx
-; MSVC-X86-O0-NEXT:    movl %esp, %eax
-; MSVC-X86-O0-NEXT:    movl %ecx, 4(%eax)
+; MSVC-X86-O0-NEXT:    movl {{[0-9]+}}(%esp), %eax
 ; MSVC-X86-O0-NEXT:    leal {{[0-9]+}}(%esp), %ecx
-; MSVC-X86-O0-NEXT:    movl %ecx, (%eax)
+; MSVC-X86-O0-NEXT:    movl %ecx, (%esp)
+; MSVC-X86-O0-NEXT:    movl %eax, {{[0-9]+}}(%esp)
 ; MSVC-X86-O0-NEXT:    calll _strcpy
 ; MSVC-X86-O0-NEXT:    leal LC, %ecx
 ; MSVC-X86-O0-NEXT:    leal {{[0-9]+}}(%esp), %eax