[llvm] [X86] Fix assertion failure for tail call on i686+pic (PR #210302)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-x86

Author: Nikita Popov (nikic)

<details>
<summary>Changes</summary>

The pattern for TCRETURNmi has a IsNotPIC predicate, but the checkTCRetEnoughRegs() predicate that's part of the X86tcret_enough_regs PatFrag is evaluated first, so we can't assert that PIC is disabled here. We should just return false in that case.

Fixes https://github.com/llvm/llvm-project/issues/210300.

---
Full diff: https://github.com/llvm/llvm-project/pull/210302.diff


2 Files Affected:

- (modified) llvm/lib/Target/X86/X86ISelDAGToDAG.cpp (+2-1) 
- (added) llvm/test/CodeGen/X86/tailcall-i686-pic.ll (+14) 


``````````diff
diff --git a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
index b794567f6a047..9a0045367a8bf 100644
--- a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
+++ b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
@@ -3561,7 +3561,8 @@ bool X86DAGToDAGISel::checkTCRetEnoughRegs(SDNode *N) const {
       LoadGPRs -= 2; // Base is fixed index off ESP; no regs needed.
     } else if (BasePtr.getOpcode() == X86ISD::Wrapper &&
                isa<GlobalAddressSDNode>(BasePtr->getOperand(0))) {
-      assert(!getTargetMachine().isPositionIndependent());
+      if (getTargetMachine().isPositionIndependent())
+        return false;
       LoadGPRs -= 1; // Base is a global (immediate since this is non-PIC), no
                      // reg needed.
     }
diff --git a/llvm/test/CodeGen/X86/tailcall-i686-pic.ll b/llvm/test/CodeGen/X86/tailcall-i686-pic.ll
new file mode 100644
index 0000000000000..b8a4207451e8a
--- /dev/null
+++ b/llvm/test/CodeGen/X86/tailcall-i686-pic.ll
@@ -0,0 +1,14 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 6
+; RUN: llc -mtriple=i686-pc-windows-msvc -relocation-model=pic < %s | FileCheck %s
+
+ at fnptr = external global ptr
+
+define void @test() {
+; CHECK-LABEL: test:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movl _fnptr, %eax
+; CHECK-NEXT:    jmpl *%eax # TAILCALL
+  %p = load ptr, ptr @fnptr
+  tail call void %p()
+  ret void
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/210302