[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
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 @@
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: