[llvm] [MIPS][ISel] Fix musttail (PR #161860)

[Djordje Todorovic via llvm-commits]

https://github.com/djtodoro created https://github.com/llvm/llvm-project/pull/161860

Properly handle clang::musttail attribute on MIPS backend.

It fixes: https://github.com/llvm/llvm-project/issues/161193


>From ac9497ed3fffb45a8044c043f703487baeedb4e5 Mon Sep 17 00:00:00 2001
From: Djordje Todorovic <djordje.todorovic at htecgroup.com>
Date: Fri, 3 Oct 2025 17:03:52 +0200
Subject: [PATCH] [MIPS][ISel] Fix musttail

Properly handle clang::musttail attribute on MIPS backend.
---
 llvm/lib/Target/Mips/MipsISelLowering.cpp |  7 +++--
 llvm/test/CodeGen/Mips/musttail.ll        | 38 +++++++++++++++++++++++
 2 files changed, 42 insertions(+), 3 deletions(-)
 create mode 100644 llvm/test/CodeGen/Mips/musttail.ll

diff --git a/llvm/lib/Target/Mips/MipsISelLowering.cpp b/llvm/lib/Target/Mips/MipsISelLowering.cpp
index 881ba8e2f9eff..d79db127f01a2 100644
--- a/llvm/lib/Target/Mips/MipsISelLowering.cpp
+++ b/llvm/lib/Target/Mips/MipsISelLowering.cpp
@@ -3407,7 +3407,8 @@ MipsTargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
   // Check if it's really possible to do a tail call. Restrict it to functions
   // that are part of this compilation unit.
   bool InternalLinkage = false;
-  if (IsTailCall) {
+  bool IsMustTail = CLI.CB && CLI.CB->isMustTailCall();
+  if (IsTailCall && !IsMustTail) {
     IsTailCall = isEligibleForTailCallOptimization(
         CCInfo, StackSize, *MF.getInfo<MipsFunctionInfo>());
     if (GlobalAddressSDNode *G = dyn_cast<GlobalAddressSDNode>(Callee)) {
@@ -3416,9 +3417,9 @@ MipsTargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
                      G->getGlobal()->hasPrivateLinkage() ||
                      G->getGlobal()->hasHiddenVisibility() ||
                      G->getGlobal()->hasProtectedVisibility());
-     }
+    }
   }
-  if (!IsTailCall && CLI.CB && CLI.CB->isMustTailCall())
+  if (!IsTailCall && IsMustTail)
     report_fatal_error("failed to perform tail call elimination on a call "
                        "site marked musttail");
 
diff --git a/llvm/test/CodeGen/Mips/musttail.ll b/llvm/test/CodeGen/Mips/musttail.ll
new file mode 100644
index 0000000000000..d5f457f6eb665
--- /dev/null
+++ b/llvm/test/CodeGen/Mips/musttail.ll
@@ -0,0 +1,38 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=mips-unknown-linux-gnu < %s | FileCheck %s --check-prefix=MIPS32
+; RUN: llc -mtriple=mips64-unknown-linux-gnu < %s | FileCheck %s --check-prefix=MIPS64
+
+; Test musttail support for MIPS
+
+declare void @external_func()
+
+; Test basic musttail with external function
+define void @test_musttail_external() {
+; MIPS32-LABEL: test_musttail_external:
+; MIPS32:       # %bb.0:
+; MIPS32-NEXT:    j external_func
+; MIPS32-NEXT:    nop
+;
+; MIPS64-LABEL: test_musttail_external:
+; MIPS64:       # %bb.0:
+; MIPS64-NEXT:    j external_func
+; MIPS64-NEXT:    nop
+  musttail call void @external_func()
+  ret void
+}
+
+declare i32 @callee_args(i32 %a, i32 %b, i32 %c)
+
+define i32 @test_musttail_args(i32 %x, i32 %y, i32 %z) {
+; MIPS32-LABEL: test_musttail_args:
+; MIPS32:       # %bb.0:
+; MIPS32-NEXT:    j callee_args
+; MIPS32-NEXT:    nop
+;
+; MIPS64-LABEL: test_musttail_args:
+; MIPS64:       # %bb.0:
+; MIPS64-NEXT:    j callee_args
+; MIPS64-NEXT:    nop
+  %ret = musttail call i32 @callee_args(i32 %x, i32 %y, i32 %z)
+  ret i32 %ret
+}