[PATCH] D29020: [ARM] Change TCReturn to tBL if tailcall optimization fails.

[Sanne Wouda via Phabricator via llvm-commits]

sanwou01 updated this revision to Diff 86650.
sanwou01 edited the summary of this revision.
sanwou01 added a comment.

Updated patch re @olista01 comments.


https://reviews.llvm.org/D29020

Files:
  lib/Target/ARM/ARMSubtarget.cpp
  lib/Target/ARM/Thumb1FrameLowering.cpp
  test/CodeGen/ARM/v8m-tail-call.ll


Index: test/CodeGen/ARM/v8m-tail-call.ll
===================================================================
--- /dev/null
+++ test/CodeGen/ARM/v8m-tail-call.ll
@@ -0,0 +1,23 @@
+; RUN: llc %s -o - -mtriple=thumbv8m.base | FileCheck %s
+
+define void @test() {
+; CHECK-LABEL: test:
+entry:
+  %call = tail call i32 @foo()
+  %tail = tail call i32 @foo()
+  ret void
+; CHECK: bl foo
+; CHECK: bl foo
+; CHECK-NOT: b foo
+}
+
+define void @test2() {
+; CHECK-LABEL: test2:
+entry:
+  %tail = tail call i32 @foo()
+  ret void
+; CHECK: b foo
+; CHECK-NOT: bl foo
+}
+
+declare i32 @foo()
Index: lib/Target/ARM/Thumb1FrameLowering.cpp
===================================================================
--- lib/Target/ARM/Thumb1FrameLowering.cpp
+++ lib/Target/ARM/Thumb1FrameLowering.cpp
@@ -866,6 +866,16 @@
         // ARMv4T requires BX, see emitEpilogue
         if (!STI.hasV5TOps())
           continue;
+        // Tailcall optimization failed; change TCRETURN to a tBL
+        if (MI->getOpcode() == ARM::TCRETURNdi ||
+            MI->getOpcode() == ARM::TCRETURNri) {
+          unsigned Opcode = MI->getOpcode() == ARM::TCRETURNdi
+                            ? ARM::tBL : ARM::tBLXr;
+          MachineInstrBuilder BL = BuildMI(MF, DL, TII.get(Opcode));
+          BL.add(predOps(ARMCC::AL));
+          BL.add(MI->getOperand(0));
+          MBB.insert(MI, &*BL);
+        }
         Reg = ARM::PC;
         (*MIB).setDesc(TII.get(ARM::tPOP_RET));
         if (MI != MBB.end())
Index: lib/Target/ARM/ARMSubtarget.cpp
===================================================================
--- lib/Target/ARM/ARMSubtarget.cpp
+++ lib/Target/ARM/ARMSubtarget.cpp
@@ -200,12 +200,12 @@
   // support in the assembler and linker to be used. This would need to be
   // fixed to fully support tail calls in Thumb1.
   //
-  // Doing this is tricky, since the LDM/POP instruction on Thumb doesn't take
-  // LR.  This means if we need to reload LR, it takes an extra instructions,
-  // which outweighs the value of the tail call; but here we don't know yet
-  // whether LR is going to be used.  Probably the right approach is to
-  // generate the tail call here and turn it back into CALL/RET in
-  // emitEpilogue if LR is used.
+  // For ARMv8-M, we /do/ implement tail calls.  Doing this is tricky for v8-M
+  // baseline, since the LDM/POP instruction on Thumb doesn't take LR.  This
+  // means if we need to reload LR, it takes an extra instructions, which
+  // outweighs the value of the tail call; but here we don't know yet whether LR
+  // is going to be used. We generate generate the tail call here and turn it
+  // back into CALL/RET in emitEpilogue if LR is used.
 
   // Thumb1 PIC calls to external symbols use BX, so they can be tail calls,
   // but we need to make sure there are enough registers; the only valid


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