[llvm] 38f3d0b - [MCP] Never eliminate frame-setup/destroy instructions (#186237)

[via llvm-commits]

Author: Scott Linder
Date: 2026-04-16T15:00:32-04:00
New Revision: 38f3d0be121c04b42f0988a25c22ea67543919f8

URL: https://github.com/llvm/llvm-project/commit/38f3d0be121c04b42f0988a25c22ea67543919f8
DIFF: https://github.com/llvm/llvm-project/commit/38f3d0be121c04b42f0988a25c22ea67543919f8.diff

LOG: [MCP] Never eliminate frame-setup/destroy instructions (#186237)

Presumably targets only insert frame instructions which are significant,
and there may be effects MCP doesn't model. Similar to reserved
registers this
is probably overly conservative, but as this causes no codegen change in
any lit test I think it is benign.

The motivation is just to clean up #183149 for AMDGPU, as we can spill
to physical registers, and currently have to spill the EXEC mask purely
to enable debug-info.

Change-Id: I9ea4a09b34464c43322edd2900361bf635efd9f7

Added: 
    

Modified: 
    llvm/lib/CodeGen/MachineCopyPropagation.cpp
    llvm/test/CodeGen/X86/machine-copy-prop.mir

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/MachineCopyPropagation.cpp b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
index fbd833abc724c..4d2a66f41df93 100644
--- a/llvm/lib/CodeGen/MachineCopyPropagation.cpp
+++ b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
@@ -490,6 +490,12 @@ class MachineCopyPropagation {
     // zero).
     return MRI->isReserved(CopyOperand);
   }
+  /// Returns true iff the @p Copy instruction must never be eliminated as
+  /// redundant. This overload does not consider the operands of @p Copy.
+  bool isNeverRedundant(const MachineInstr &Copy) {
+    return Copy.getFlag(MachineInstr::FrameSetup) ||
+           Copy.getFlag(MachineInstr::FrameDestroy);
+  }
   bool hasImplicitOverlap(const MachineInstr &MI, const MachineOperand &Use);
   bool hasOverlappingMultipleDef(const MachineInstr &MI,
                                  const MachineOperand &MODef, MCRegister Def);
@@ -598,7 +604,7 @@ static bool isNopCopy(const MachineInstr &PreviousCopy, MCRegister Src,
 /// copying the super registers.
 bool MachineCopyPropagation::eraseIfRedundant(MachineInstr &Copy,
                                               MCRegister Dst, MCRegister Src) {
-  if (isNeverRedundant(Src) || isNeverRedundant(Dst))
+  if (isNeverRedundant(Copy) || isNeverRedundant(Src) || isNeverRedundant(Dst))
     return false;
 
   // Search for an existing copy.
@@ -659,7 +665,7 @@ bool MachineCopyPropagation::isBackwardPropagatableCopy(
   if (!Dst || !Src)
     return false;
 
-  if (isNeverRedundant(Dst) || isNeverRedundant(Src))
+  if (isNeverRedundant(Copy) || isNeverRedundant(Dst) || isNeverRedundant(Src))
     return false;
 
   return CopyOperands.Source->isRenamable() && CopyOperands.Source->isKill();
@@ -946,7 +952,7 @@ void MachineCopyPropagation::forwardCopyPropagateBlock(MachineBasicBlock &MBB) {
       if (!TRI->regsOverlap(Dst, Src)) {
         // FIXME: Document why this does not consider `RegSrc`, similar to how
         // `backwardCopyPropagateBlock` does.
-        if (!isNeverRedundant(Dst))
+        if (!isNeverRedundant(MI) && !isNeverRedundant(Dst))
           MaybeDeadCopies.insert(&MI);
       }
     }
@@ -991,7 +997,7 @@ void MachineCopyPropagation::forwardCopyPropagateBlock(MachineBasicBlock &MBB) {
         std::optional<DestSourcePair> CopyOperands =
             isCopyInstr(*MaybeDead, *TII, UseCopyInstr);
         MCRegister Reg = CopyOperands->Destination->getReg().asMCReg();
-        assert(!isNeverRedundant(Reg));
+        assert(!isNeverRedundant(*MaybeDead) && !isNeverRedundant(Reg));
 
         if (!RegMask->clobbersPhysReg(Reg)) {
           ++DI;
@@ -1059,7 +1065,7 @@ void MachineCopyPropagation::forwardCopyPropagateBlock(MachineBasicBlock &MBB) {
           *isCopyInstr(*MaybeDead, *TII, UseCopyInstr);
 
       auto [Dst, Src] = getDstSrcMCRegs(CopyOperands);
-      assert(!isNeverRedundant(Dst));
+      assert(!isNeverRedundant(*MaybeDead) && !isNeverRedundant(Dst));
 
       // Update matching debug values, if any.
       const auto &DbgUsers = CopyDbgUsers[MaybeDead];

diff  --git a/llvm/test/CodeGen/X86/machine-copy-prop.mir b/llvm/test/CodeGen/X86/machine-copy-prop.mir
index f4fed03a75e67..e04d638ef092a 100644
--- a/llvm/test/CodeGen/X86/machine-copy-prop.mir
+++ b/llvm/test/CodeGen/X86/machine-copy-prop.mir
@@ -16,6 +16,8 @@
   define void @nocopyprop3() { ret void }
   define void @nocopyprop4() { ret void }
   define void @nocopyprop5() { ret void }
+  define void @nocopyprop_frame_setup() { ret void }
+  define void @nocopyprop_frame_destroy() { ret void }
 ...
 ---
 # The second copy is redundant and will be removed, check that we also remove
@@ -253,3 +255,23 @@ body: |
     $rip = COPY $rax
     $rip = COPY $rax
 ...
+---
+# Conservatively assume prologue/epilogue instructions may have
+# additional effects.
+# CHECK-LABEL: name: nocopyprop_frame_setup
+# CHECK: bb.0:
+# CHECK-NEXT: $rbx = frame-setup COPY $rax
+name: nocopyprop_frame_setup
+body: |
+  bb.0:
+    $rbx = frame-setup COPY $rax
+...
+---
+# CHECK-LABEL: name: nocopyprop_frame_destroy
+# CHECK: bb.0:
+# CHECK-NEXT: $rbx = frame-destroy COPY $rax
+name: nocopyprop_frame_destroy
+body: |
+  bb.0:
+    $rbx = frame-destroy COPY $rax
+...