Short-circuiting lvrx/stvrx. Should help bad accesses in many games.

Fixes #411.

src/xenia/cpu/frontend/ppc_emit_altivec.cc

@@ -245,14 +245,26 @@ XEEMITTER(lvlxl128, VX128_1(4, 1539), VX128_1)(PPCHIRBuilder& f, InstrData& i) {
 
 int InstrEmit_lvrx_(PPCHIRBuilder& f, InstrData& i, uint32_t vd, uint32_t ra,
                     uint32_t rb) {
+  // NOTE: if eb == 0 (so 16b aligned) then no data is loaded. This is important
+  // as often times memcpy's will use this to handle the remaining <=16b of a
+  // buffer, which sometimes may be nothing and hang off the end of the valid
+  // page area. We still need to zero the resulting register, though.
   Value* ea = CalculateEA_0(f, ra, rb);
   Value* eb = f.And(f.Truncate(ea, INT8_TYPE), f.LoadConstantInt8(0xF));
+  // Skip if %16=0 (just load zero).
+  auto load_label = f.NewLabel();
+  auto end_label = f.NewLabel();
+  f.BranchTrue(eb, load_label);
+  f.StoreVR(vd, f.LoadZeroVec128());
+  f.Branch(end_label);
+  f.MarkLabel(load_label);
   // ea &= ~0xF
   ea = f.And(ea, f.LoadConstantUint64(~0xFull));
   // v = (new >> (16 - eb))
   Value* v = f.Permute(f.LoadVectorShl(eb), f.LoadZeroVec128(),
                        f.ByteSwap(f.Load(ea, VEC128_TYPE)), INT8_TYPE);
   f.StoreVR(vd, v);
+  f.MarkLabel(end_label);
   return 0;
 }
 XEEMITTER(lvrx, 0x7C00044E, X)(PPCHIRBuilder& f, InstrData& i) {
@@ -304,10 +316,15 @@ XEEMITTER(stvlxl128, VX128_1(4, 1795), VX128_1)(PPCHIRBuilder& f,
 
 int InstrEmit_stvrx_(PPCHIRBuilder& f, InstrData& i, uint32_t vd, uint32_t ra,
                      uint32_t rb) {
-  // NOTE: if eb == 0 (so 16b aligned) this equals new_value
-  //       we could optimize this to prevent the other load/mask, in that case.
+  // NOTE: if eb == 0 (so 16b aligned) then no data is loaded. This is important
+  // as often times memcpy's will use this to handle the remaining <=16b of a
+  // buffer, which sometimes may be nothing and hang off the end of the valid
+  // page area.
   Value* ea = CalculateEA_0(f, ra, rb);
   Value* eb = f.And(f.Truncate(ea, INT8_TYPE), f.LoadConstantInt8(0xF));
+  // Skip if %16=0 (no data to store).
+  auto skip_label = f.NewLabel();
+  f.BranchFalse(eb, skip_label);
   // ea &= ~0xF
   ea = f.And(ea, f.LoadConstantUint64(~0xFull));
   // v = (old & ~mask) | ((new << eb) & mask)
@@ -320,6 +337,7 @@ int InstrEmit_stvrx_(PPCHIRBuilder& f, InstrData& i, uint32_t vd, uint32_t ra,
   Value* v = f.Or(f.And(old_value, f.Not(mask)), f.And(new_value, mask));
   // ea &= ~0xF (handled above)
   f.Store(ea, f.ByteSwap(v));
+  f.MarkLabel(skip_label);
   return 0;
 }
 XEEMITTER(stvrx, 0x7C00054E, X)(PPCHIRBuilder& f, InstrData& i) {

src/xenia/cpu/frontend/testing/instr_lvr.s

@@ -18,3 +18,23 @@ test_lvr_1_constant:
   #_ REGISTER_OUT r4 0x100010B7
   #_ REGISTER_OUT r5 0x10
   #_ REGISTER_OUT v3 [00000000, 00000000, 000D0E10, 11121314]
+
+test_lvr_2:
+  #_ REGISTER_IN r4 0x20000000
+  #_ REGISTER_IN r5 0x10
+  #_ REGISTER_IN v3 [FFFFFFFF, FFFFFFFF, FFFFFFFF, FFFFFFFF]
+  lvrx v3, r4, r5
+  blr
+  #_ REGISTER_OUT r4 0x20000000
+  #_ REGISTER_OUT r5 0x10
+  #_ REGISTER_OUT v3 [00000000, 00000000, 00000000, 00000000]
+
+test_lvr_2_constant:
+  #_ REGISTER_IN v3 [FFFFFFFF, FFFFFFFF, FFFFFFFF, FFFFFFFF]
+  lis r4, 0x2000
+  li r5, 0x10
+  lvrx v3, r4, r5
+  blr
+  #_ REGISTER_OUT r4 0x20000000
+  #_ REGISTER_OUT r5 0x10
+  #_ REGISTER_OUT v3 [00000000, 00000000, 00000000, 00000000]

src/xenia/cpu/frontend/testing/instr_stvr.s

@@ -47,3 +47,21 @@ test_stvr_2_constant:
   #_ REGISTER_OUT r4 0x10001044
   #_ REGISTER_OUT v3 [F0F1F2F3, F4F5F6F7, F8F9FAFB, FCFDFEFF]
   #_ MEMORY_OUT 10001040 FCFDFEFF 04050607 08090A0B 0C0D0E0F
+
+test_stvr_3:
+  #_ REGISTER_IN r4 0x10010000
+  #_ REGISTER_IN r5 0x0
+  #_ REGISTER_IN v3 [BE74FCBD, BD912ABA, BF317BBB, BF2D135F]
+  stvrx v3, r4, r5
+  blr
+  #_ REGISTER_OUT r4 0x10010000
+  #_ REGISTER_OUT r5 0x0
+  #_ REGISTER_OUT v3 [BE74FCBD, BD912ABA, BF317BBB, BF2D135F]
+
+test_stvr_3_constant:
+  #_ REGISTER_IN v3 [BE74FCBD, BD912ABA, BF317BBB, BF2D135F]
+  lis r4, 0x1001
+  stvrx v3, r4, r0
+  blr
+  #_ REGISTER_OUT r4 0x10010000
+  #_ REGISTER_OUT v3 [BE74FCBD, BD912ABA, BF317BBB, BF2D135F]