[ARM] Refactor the prologue/epilogue emission to be more robust.

This is the first step toward supporting shrink-wrapping for this target.

The changes could be summarized by these items:
- Expand the tail-call return as part of the expand pseudo pass.
- Get rid of the assumptions that the epilogue is the exit block:
  * Do not assume which registers are free in the epilogue. (This indirectly
    improve the lowering of the code for the segmented stacks, see the test
    cases.)
  * Take into account that the basic block can be empty.

Related to <rdar://problem/20821730>

llvm-svn: 242714

2 files changed, 5 insertions, 5 deletions

diff --git a/llvm/test/CodeGen/Thumb/pop.ll b/llvm/test/CodeGen/Thumb/pop.ll
index 3c539c69017..851f793e5ee 100644
--- a/llvm/test/CodeGen/Thumb/pop.ll
+++ b/llvm/test/CodeGen/Thumb/pop.ll
@@ -3,9 +3,9 @@
 
 define void @t(i8* %a, ...) nounwind {
 ; CHECK-LABEL:      t:
-; CHECK:      pop {r3}
+; CHECK:      pop {[[POP_REG:r[0-3]]]}
 ; CHECK-NEXT: add sp, #12
-; CHECK-NEXT: bx r3
+; CHECK-NEXT: bx [[POP_REG]]
 entry:
   %a.addr = alloca i8, i32 4
   call void @llvm.va_start(i8* %a.addr)
diff --git a/llvm/test/CodeGen/Thumb/vargs.ll b/llvm/test/CodeGen/Thumb/vargs.ll
index 1c7b631741b..45b42125e16 100644
--- a/llvm/test/CodeGen/Thumb/vargs.ll
+++ b/llvm/test/CodeGen/Thumb/vargs.ll
@@ -32,12 +32,12 @@ bb7:            ; preds = %bb
         call void @llvm.va_end( i8* %va.upgrd.4 )
         ret void
 
-; The return sequence should pop the lr to r3, recover the stack space used to
+; The return sequence should pop the lr to r0-3, recover the stack space used to
 ; store variadic argument registers, then return via r3. Possibly there is a pop
 ; before this, but only if the function happened to use callee-saved registers.
-; CHECK: pop {r3}
+; CHECK: pop {[[POP_REG:r[0-3]]]}
 ; CHECK: add sp, #[[IMM]]
-; CHECK: bx r3
+; CHECK: bx [[POP_REG]]
 }
 
 declare void @llvm.va_start(i8*)