AMDGPU: Make FrameLowering stack alignment 16

[arsenm]

Summary

We don't need it to be that high. The natural alignment for a single workitem's stack is 16.

[• tstellarAMD]

LGTM.

[arsenm]

r273448

lib/Target/AMDGPU/AMDGPUSubtarget.cpp

: AMDGPUGenSubtargetInfo(TT, GPU, FS),
       EnableSIScheduler(false),
       DebuggerInsertNops(false), DebuggerReserveRegs(false),
       FrameLowering(nullptr),
       GISel(),
       InstrItins(getInstrItineraryForCPU(GPU)), TargetTriple(TT) {
 
   initializeSubtargetDependencies(TT, GPU, FS);
 
-  const unsigned MaxStackAlign = 64 * 16; // Maximum stack alignment (long16)
+  // Scratch is allocated in 256 dword per wave blocks.
+  const unsigned StackAlign = 4 * 256 / getWavefrontSize();
 
   if (getGeneration() <= AMDGPUSubtarget::NORTHERN_ISLANDS) {
     InstrInfo.reset(new R600InstrInfo(*this));
     TLInfo.reset(new R600TargetLowering(TM, *this));
 
     // FIXME: Should have R600 specific FrameLowering
     FrameLowering.reset(new AMDGPUFrameLowering(
                           TargetFrameLowering::StackGrowsUp,
-                          MaxStackAlign,
+                          StackAlign,
                           0));
   } else {
     InstrInfo.reset(new SIInstrInfo(*this));
     TLInfo.reset(new SITargetLowering(TM, *this));
     FrameLowering.reset(new SIFrameLowering(
                           TargetFrameLowering::StackGrowsUp,
-                          MaxStackAlign,
+                          StackAlign,
                           0));
 #ifndef LLVM_BUILD_GLOBAL_ISEL
     GISelAccessor *GISel = new GISelAccessor();
 #else
     AMDGPUGISelActualAccessor *GISel =
         new AMDGPUGISelActualAccessor();
     GISel->CallLoweringInfo.reset(
         new AMDGPUCallLowering(*getTargetLowering()));