[llvm] 071b867 - [Mips] Did not put fake_use into delay slot (#201537)

[via llvm-commits]

Author: yingopq
Date: 2026-07-16T10:19:25+08:00
New Revision: 071b86797f62e827bf18e83667b0d1875a5ee446

URL: https://github.com/llvm/llvm-project/commit/071b86797f62e827bf18e83667b0d1875a5ee446
DIFF: https://github.com/llvm/llvm-project/commit/071b86797f62e827bf18e83667b0d1875a5ee446.diff

LOG: [Mips] Did not put fake_use into delay slot (#201537)

When compiling with -fextend-variable-liveness, the compiler generates FAKE_USE instructions that are not handled by the Mips assembly printer. This causes a fatal error: "Unsupported instruction : <MCInst 44 >" when using clang with -target mipsel-gnu-linux -Og.

The issue occurs because MipsDelaySlotFiller attempts to process FAKE_USE instructions (which are meta instructions) when searching for instructions to fill delay slots, eventually leading to the assembly printer trying to emit an unsupported instruction.

This patch fixes the crash by treating meta instructions (including FAKE_USE) the same way as debug instructions in the delay slot filler's searchRange function. Meta instructions should be skipped during delay slot filling.

Specifically, the fix:
1.Moves the terminateSearch check before debug/meta instruction checks to ensure proper search termination.
2.Adds isMetaInstruction() check alongside existing isDebugInstr() and isJumpTableDebugInfo() checks.

Fix #198835.

Added: 
    llvm/test/CodeGen/Mips/fake-use.ll

Modified: 
    llvm/lib/Target/Mips/MipsDelaySlotFiller.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Target/Mips/MipsDelaySlotFiller.cpp b/llvm/lib/Target/Mips/MipsDelaySlotFiller.cpp
index bcfaa7cd2ed2d..7d7dac22520ea 100644
--- a/llvm/lib/Target/Mips/MipsDelaySlotFiller.cpp
+++ b/llvm/lib/Target/Mips/MipsDelaySlotFiller.cpp
@@ -854,10 +854,17 @@ bool MipsDelaySlotFiller::searchRange(MachineBasicBlock &MBB, IterTy Begin,
     IterTy CurrI = I;
     ++I;
     LLVM_DEBUG(dbgs() << DEBUG_TYPE ": checking instruction: "; CurrI->dump());
-    // Skip debug value.
+
+    if (terminateSearch(*CurrI)) {
+      LLVM_DEBUG(dbgs() << DEBUG_TYPE ": should terminate search: ";
+                 CurrI->dump());
+      break;
+    }
+    // Skip debug and pseudo instructions.
     // Instruction TargetOpcode::JUMP_TABLE_DEBUG_INFO is only used to note
     // jump table debug info.
-    if (CurrI->isDebugInstr() || CurrI->isJumpTableDebugInfo()) {
+    if (CurrI->isDebugInstr() || CurrI->isJumpTableDebugInfo() ||
+        CurrI->isMetaInstruction()) {
       LLVM_DEBUG(dbgs() << DEBUG_TYPE ": ignoring debug instruction: ";
                  CurrI->dump());
       continue;
@@ -871,12 +878,6 @@ bool MipsDelaySlotFiller::searchRange(MachineBasicBlock &MBB, IterTy Begin,
       continue;
     }
 
-    if (terminateSearch(*CurrI)) {
-      LLVM_DEBUG(dbgs() << DEBUG_TYPE ": should terminate search: ";
-                 CurrI->dump());
-      break;
-    }
-
     assert((!CurrI->isCall() && !CurrI->isReturn() && !CurrI->isBranch()) &&
            "Cannot put calls, returns or branches in delay slot.");
 

diff  --git a/llvm/test/CodeGen/Mips/fake-use.ll b/llvm/test/CodeGen/Mips/fake-use.ll
new file mode 100644
index 0000000000000..c8c7f83624314
--- /dev/null
+++ b/llvm/test/CodeGen/Mips/fake-use.ll
@@ -0,0 +1,15 @@
+; RUN: llc -verify-machineinstrs -mtriple=mipsel-linux-gnu -mcpu=mips32r2 %s -o - | FileCheck %s
+
+define void @fake_use(ptr %aaaa) {
+; CHECK-LABEL: fake_use:
+; CHECK:       # %bb.0:    # %entry
+; CHECK-NEXT:              # fake_use: $a0 
+; CHECK-NEXT:    jr $ra 
+; CHECK-NEXT:    nop
+entry:
+  notail call void (...) @llvm.fake.use(ptr %aaaa)
+  ret void
+}
+
+declare void @llvm.fake.use(...)
+