[ARM] Avoid redundant checks for isThumb1Only() after supportsTailCall()

supportsTailCall() has two callers. Both of them double-check isThumb1Only(),
and refuse to proceed with tail-calling in that case.
Therefore, it makes sense to move this check to
ARMSubtarget::initSubtargetFeatures, where SupportsTailCall is initialized;
and to eliminate the extra checks at the call sites.

Following a review comment, added an "assert(supportsTailCall())"
in IsEligibleForTailCall.

NFC.

llvm-svn: 248703

1 files changed, 22 insertions, 4 deletions

diff --git a/llvm/lib/Target/ARM/ARMSubtarget.cpp b/llvm/lib/Target/ARM/ARMSubtarget.cpp
index aff38c0..e0d9939 100644
--- a/llvm/lib/Target/ARM/ARMSubtarget.cpp
+++ b/llvm/lib/Target/ARM/ARMSubtarget.cpp
@@ -193,10 +193,28 @@ void ARMSubtarget::initSubtargetFeatures(StringRef CPU, StringRef FS) {
   if (isTargetNaCl())
     stackAlignment = 16;
 
-  if (isTargetMachO())
-    SupportsTailCall = !isTargetIOS() || !getTargetTriple().isOSVersionLT(5, 0);
-  else
-    SupportsTailCall = !isThumb1Only();
+  // FIXME: Completely disable sibcall for Thumb1 since ThumbRegisterInfo::
+  // emitEpilogue is not ready for them. Thumb tail calls also use t2B, as
+  // the Thumb1 16-bit unconditional branch doesn't have sufficient relocation
+  // 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.
+
+  // 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
+  // registers are the 4 used for parameters.  We don't currently do this
+  // case.
+
+  SupportsTailCall = !isThumb1Only();
+
+  if (isTargetMachO() && isTargetIOS() && getTargetTriple().isOSVersionLT(5, 0))
+    SupportsTailCall = false;
 
   switch (IT) {
   case DefaultIT: