[PowerPC] Account for the ELFv2 function prologue during branch selection

The PPC branch-selection pass, which performs branch relaxation, needs to
account for the padding that might be introduced to satisfy block alignment
requirements. We were assuming that the first block was at offset zero (i.e.
had the alignment of the function itself), but under the ELFv2 ABI, a global
entry function prologue is added to the first block, and it is a
two-instruction sequence (i.e. eight-bytes long). If the function has 16-byte
alignment, the fact that the first block is eight bytes offset from the start
of the function is relevant to calculating where padding will be added in
between later blocks.

Unfortunately, I don't have a small test case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@283086 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/PowerPC/PPCAsmPrinter.cpp

@@ -1199,6 +1199,9 @@ void PPCLinuxAsmPrinter::EmitFunctionBodyStart() {
   if (Subtarget->isELFv2ABI()
       // Only do all that if the function uses r2 in the first place.
       && !MF->getRegInfo().use_empty(PPC::X2)) {
+    // Note: The logic here must be synchronized with the code in the
+    // branch-selection pass which sets the offset of the first block in the
+    // function. This matters because it affects the alignment.
     const PPCFunctionInfo *PPCFI = MF->getInfo<PPCFunctionInfo>();
 
     MCSymbol *GlobalEntryLabel = PPCFI->getGlobalEPSymbol();

lib/Target/PowerPC/PPCBranchSelector.cpp

@@ -19,8 +19,10 @@
 #include "MCTargetDesc/PPCPredicates.h"
 #include "PPCInstrBuilder.h"
 #include "PPCInstrInfo.h"
+#include "PPCSubtarget.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/Target/TargetSubtargetInfo.h"
@@ -92,8 +94,19 @@ bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
     return AlignAmt + OffsetToAlignment(Offset, AlignAmt);
   };
 
+  // We need to be careful about the offset of the first block in the function
+  // because it might not have the function's alignment. This happens because,
+  // under the ELFv2 ABI, for functions which require a TOC pointer, we add a
+  // two-instruction sequence to the start of the function.
+  // Note: This needs to be synchronized with the check in
+  // PPCLinuxAsmPrinter::EmitFunctionBodyStart.
+  unsigned InitialOffset = 0;
+  if (Fn.getSubtarget<PPCSubtarget>().isELFv2ABI() &&
+      !Fn.getRegInfo().use_empty(PPC::X2))
+    InitialOffset = 8;
+
   // Measure each MBB and compute a size for the entire function.
-  unsigned FuncSize = 0;
+  unsigned FuncSize = InitialOffset;
   for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
        ++MFI) {
     MachineBasicBlock *MBB = &*MFI;
@@ -240,7 +253,7 @@ bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
     if (MadeChange) {
       // If we're going to iterate again, make sure we've updated our
       // padding-based contributions to the block sizes.
-      unsigned Offset = 0;
+      unsigned Offset = InitialOffset;
       for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
            ++MFI) {
         MachineBasicBlock *MBB = &*MFI;