From 90c8bc50504a5e4f246a8c7e0c8a8f238a822348 Mon Sep 17 00:00:00 2001
From: Tim Northover <t.p.northover@gmail.com>
Date: Mon, 8 Mar 2021 14:45:46 +0000
Subject: [PATCH] FastISel: do not fold a swifterror load into anything.

Swifterror loads aren't really loads at all, but copies from the designated
swifterror register, so FastISel shouldn't try to turn them into a real load.
---
 llvm/lib/CodeGen/SelectionDAG/FastISel.cpp |  6 ++++++
 llvm/test/CodeGen/X86/swifterror-O0.ll     | 20 ++++++++++++++++++++
 2 files changed, 26 insertions(+)
 create mode 100644 llvm/test/CodeGen/X86/swifterror-O0.ll

diff --git a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
index d5cc8dc6608df..aad963582e3d5 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -2413,6 +2413,12 @@ bool FastISel::tryToFoldLoad(const LoadInst *LI, const Instruction *FoldInst) {
   if (LI->isVolatile())
     return false;
 
+  // Swifterror loads are not actually loads.
+  if (auto AI = dyn_cast<AllocaInst>(LI->getPointerOperand())) {
+    if (AI->isSwiftError())
+      return false;
+  }
+
   // Figure out which vreg this is going into.  If there is no assigned vreg yet
   // then there actually was no reference to it.  Perhaps the load is referenced
   // by a dead instruction.
diff --git a/llvm/test/CodeGen/X86/swifterror-O0.ll b/llvm/test/CodeGen/X86/swifterror-O0.ll
new file mode 100644
index 0000000000000..5dd1bbd0572be
--- /dev/null
+++ b/llvm/test/CodeGen/X86/swifterror-O0.ll
@@ -0,0 +1,20 @@
+; RUN: llc -mtriple=x86_64-apple-darwin -O0 %s -o - | FileCheck %s
+
+define i32 @foo() {
+; CHECK-LABEL: foo:
+; CHECK: cmpq $0, %r12
+
+  %swifterror = alloca swifterror i8*
+  call void @callee(i8** swifterror %swifterror)
+  %err = load i8*, i8** %swifterror
+  %tst = icmp ne i8* %err, null
+  br i1 %tst, label %true, label %false
+
+true:
+  ret i32 0
+
+false:
+  ret i32 1
+}
+
+declare void @callee(i8** swifterror)