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

[Nikita Popov via llvm-commits]

https://github.com/nikic created https://github.com/llvm/llvm-project/pull/171782

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

Looking at https://github.com/llvm/llvm-project/commit/3e6fa462f3c617ca8202162da2a3990c97b9ec36, 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 https://github.com/llvm/llvm-project/pull/171288.

>From 63e86a3ca3f3899981a5f5ea8201712e76aba0e1 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Thu, 11 Dec 2025 09:47:43 +0100
Subject: [PATCH] [FastISel] Don't force SDAG fallback for libcalls

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

Note that clang will already directly emit intrinsics for most
of these anyway, so there should be little change in practice.
---
 llvm/lib/CodeGen/SelectionDAG/FastISel.cpp       | 8 --------
 llvm/test/CodeGen/X86/stack-protector-msvc-oz.ll | 7 +++----
 llvm/test/CodeGen/X86/stack-protector-msvc.ll    | 7 +++----
 3 files changed, 6 insertions(+), 16 deletions(-)

diff --git a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
index 5c84059da273b..51391f1aeecde 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/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 &&
diff --git a/llvm/test/CodeGen/X86/stack-protector-msvc-oz.ll b/llvm/test/CodeGen/X86/stack-protector-msvc-oz.ll
index d8a772efbd7ed..4ab23cdde74c6 100644
--- a/llvm/test/CodeGen/X86/stack-protector-msvc-oz.ll
+++ b/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
diff --git a/llvm/test/CodeGen/X86/stack-protector-msvc.ll b/llvm/test/CodeGen/X86/stack-protector-msvc.ll
index a868fa549296d..3109733e0b0b7 100644
--- a/llvm/test/CodeGen/X86/stack-protector-msvc.ll
+++ b/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