[gba] cpu: Do not postpone timer IRQs, fixes nba-emu#40.

CMakeLists.txt

@@ -9,4 +9,7 @@ set(COMPILE_FLAGS   "${COMPILE_FLAGS} -Ofast")
 set(CMAKE_C_FLAGS   "${CMAKE_C_FLAGS}   ${COMPILE_FLAGS}")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${COMPILE_FLAGS} -std=c++1z")
 
+#add_definitions(-pg)
+#set(CMAKE_EXE_LINKER_FLAGS -pg)
+
 add_subdirectory(src)

src/gba/cpu/cpu.cpp

@@ -19,6 +19,7 @@
 
 #include "cpu.hpp"
 
+#include <algorithm>
 #include <climits>
 #include <cstring>
 
@@ -143,10 +144,10 @@ void CPU::RunFor(int cycles) {
           cpu.SignalIrq();
         cpu.Run();
       } else {
-        /* TODO: inaccurate due to timer interrupts. */
-        timers.Run(ticks_cpu_left);
-        ticks_cpu_left = 0;
-        break;
+        int advance = std::min(timers.GetCyclesUntilIrq(), ticks_cpu_left);
+
+        timers.Run(advance);
+        ticks_cpu_left -= advance;
       }
     }
 

src/gba/cpu/timer.cpp

@@ -20,6 +20,8 @@
 #include "cpu.hpp"
 #include "timer.hpp"
 
+#include <climits>
+
 using namespace GameBoyAdvance;
 
 static constexpr int g_ticks_shift[4] = { 0, 6, 8, 10 };
@@ -92,6 +94,24 @@ void TimerController::RunFIFO(int id, int times) {
   }
 }
 
+auto TimerController::GetCyclesUntilIrq() -> int {
+  int cycles = INT_MAX;
+
+  for (auto const& timer : this->timer) {
+    if (timer.control.interrupt && 
+        timer.control.enable &&
+       !timer.control.cascade)
+    {
+      int required = ((0x10000 - timer.counter) << timer.shift) - timer.cycles;
+
+      if (required < cycles)
+        cycles = required;
+    }
+  }
+
+  return cycles;
+}
+
 auto TimerController::Read(int id, int offset) -> std::uint8_t {
   switch (offset) {
     case 0: {

src/gba/cpu/timer.hpp

@@ -31,6 +31,7 @@ class TimerController {
 
   void Reset();
   void Run(int cycles);
+  auto GetCyclesUntilIrq() -> int;
   auto Read(int id, int offset) -> std::uint8_t;
   void Write(int id, int offset, std::uint8_t value);
 
@@ -39,7 +40,7 @@ class TimerController {
   void RunFIFO(int id, int times); 
 
   CPU* cpu;
-
+  
   struct Timer {
     int id;