diff --git a/Makefile b/Makefile
index 57a57cd..923c70b 100644
--- a/Makefile
+++ b/Makefile
@@ -50,7 +50,7 @@ OBJCOPY = $(TOOLPREFIX)objcopy
 OBJDUMP = $(TOOLPREFIX)objdump
 
 # CFLAGS = -fno-pic -static -fno-builtin -fno-strict-aliasing -Wall -MD -ggdb -Werror -fno-omit-frame-pointer -O
-CFLAGS = -Wall -Werror
+CFLAGS = -Wall -Werror -O
 CFLAGS += -mcmodel=medany
 CFLAGS += -ffreestanding -fno-common -nostdlib -mno-relax
 CFLAGS += $(shell $(CC) -fno-stack-protector -E -x c /dev/null >/dev/null 2>&1 && echo -fno-stack-protector)
diff --git a/riscv.h b/riscv.h
index b12e5d3..c3371a4 100644
--- a/riscv.h
+++ b/riscv.h
@@ -3,7 +3,7 @@ static inline uint64
 r_mhartid()
 {
   uint64 x;
-  asm("csrr %0, mhartid" : "=r" (x) );
+  asm volatile("csrr %0, mhartid" : "=r" (x) );
   return x;
 }
 
@@ -19,14 +19,14 @@ static inline uint64
 r_mstatus()
 {
   uint64 x;
-  asm("csrr %0, mstatus" : "=r" (x) );
+  asm volatile("csrr %0, mstatus" : "=r" (x) );
   return x;
 }
 
 static inline void 
 w_mstatus(uint64 x)
 {
-  asm("csrw mstatus, %0" : : "r" (x));
+  asm volatile("csrw mstatus, %0" : : "r" (x));
 }
 
 // machine exception program counter, holds the
@@ -35,7 +35,7 @@ w_mstatus(uint64 x)
 static inline void 
 w_mepc(uint64 x)
 {
-  asm("csrw mepc, %0" : : "r" (x));
+  asm volatile("csrw mepc, %0" : : "r" (x));
 }
 
 // Supervisor Status Register, sstatus
@@ -50,14 +50,14 @@ static inline uint64
 r_sstatus()
 {
   uint64 x;
-  asm("csrr %0, sstatus" : "=r" (x) );
+  asm volatile("csrr %0, sstatus" : "=r" (x) );
   return x;
 }
 
 static inline void 
 w_sstatus(uint64 x)
 {
-  asm("csrw sstatus, %0" : : "r" (x));
+  asm volatile("csrw sstatus, %0" : : "r" (x));
 }
 
 // Supervisor Interrupt Pending
@@ -65,14 +65,14 @@ static inline uint64
 r_sip()
 {
   uint64 x;
-  asm("csrr %0, sip" : "=r" (x) );
+  asm volatile("csrr %0, sip" : "=r" (x) );
   return x;
 }
 
 static inline void 
 w_sip(uint64 x)
 {
-  asm("csrw sip, %0" : : "r" (x));
+  asm volatile("csrw sip, %0" : : "r" (x));
 }
 
 // Supervisor Interrupt Enable
@@ -83,14 +83,14 @@ static inline uint64
 r_sie()
 {
   uint64 x;
-  asm("csrr %0, sie" : "=r" (x) );
+  asm volatile("csrr %0, sie" : "=r" (x) );
   return x;
 }
 
 static inline void 
 w_sie(uint64 x)
 {
-  asm("csrw sie, %0" : : "r" (x));
+  asm volatile("csrw sie, %0" : : "r" (x));
 }
 
 // Machine-mode Interrupt Enable
@@ -101,14 +101,14 @@ static inline uint64
 r_mie()
 {
   uint64 x;
-  asm("csrr %0, mie" : "=r" (x) );
+  asm volatile("csrr %0, mie" : "=r" (x) );
   return x;
 }
 
 static inline void 
 w_mie(uint64 x)
 {
-  asm("csrw mie, %0" : : "r" (x));
+  asm volatile("csrw mie, %0" : : "r" (x));
 }
 
 // machine exception program counter, holds the
@@ -117,14 +117,14 @@ w_mie(uint64 x)
 static inline void 
 w_sepc(uint64 x)
 {
-  asm("csrw sepc, %0" : : "r" (x));
+  asm volatile("csrw sepc, %0" : : "r" (x));
 }
 
 static inline uint64
 r_sepc()
 {
   uint64 x;
-  asm("csrr %0, sepc" : "=r" (x) );
+  asm volatile("csrr %0, sepc" : "=r" (x) );
   return x;
 }
 
@@ -133,14 +133,14 @@ static inline uint64
 r_medeleg()
 {
   uint64 x;
-  asm("csrr %0, medeleg" : "=r" (x) );
+  asm volatile("csrr %0, medeleg" : "=r" (x) );
   return x;
 }
 
 static inline void 
 w_medeleg(uint64 x)
 {
-  asm("csrw medeleg, %0" : : "r" (x));
+  asm volatile("csrw medeleg, %0" : : "r" (x));
 }
 
 // Machine Interrupt Delegation
@@ -148,14 +148,14 @@ static inline uint64
 r_mideleg()
 {
   uint64 x;
-  asm("csrr %0, mideleg" : "=r" (x) );
+  asm volatile("csrr %0, mideleg" : "=r" (x) );
   return x;
 }
 
 static inline void 
 w_mideleg(uint64 x)
 {
-  asm("csrw mideleg, %0" : : "r" (x));
+  asm volatile("csrw mideleg, %0" : : "r" (x));
 }
 
 // Supervisor Trap-Vector Base Address
@@ -163,14 +163,14 @@ w_mideleg(uint64 x)
 static inline void 
 w_stvec(uint64 x)
 {
-  asm("csrw stvec, %0" : : "r" (x));
+  asm volatile("csrw stvec, %0" : : "r" (x));
 }
 
 static inline uint64
 r_stvec()
 {
   uint64 x;
-  asm("csrr %0, stvec" : "=r" (x) );
+  asm volatile("csrr %0, stvec" : "=r" (x) );
   return x;
 }
 
@@ -178,7 +178,7 @@ r_stvec()
 static inline void 
 w_mtvec(uint64 x)
 {
-  asm("csrw mtvec, %0" : : "r" (x));
+  asm volatile("csrw mtvec, %0" : : "r" (x));
 }
 
 // use riscv's sv39 page table scheme.
@@ -191,14 +191,14 @@ w_mtvec(uint64 x)
 static inline void 
 w_satp(uint64 x)
 {
-  asm("csrw satp, %0" : : "r" (x));
+  asm volatile("csrw satp, %0" : : "r" (x));
 }
 
 static inline uint64
 r_satp()
 {
   uint64 x;
-  asm("csrr %0, satp" : "=r" (x) );
+  asm volatile("csrr %0, satp" : "=r" (x) );
   return x;
 }
 
@@ -206,13 +206,13 @@ r_satp()
 static inline void 
 w_sscratch(uint64 x)
 {
-  asm("csrw sscratch, %0" : : "r" (x));
+  asm volatile("csrw sscratch, %0" : : "r" (x));
 }
 
 static inline void 
 w_mscratch(uint64 x)
 {
-  asm("csrw mscratch, %0" : : "r" (x));
+  asm volatile("csrw mscratch, %0" : : "r" (x));
 }
 
 // Supervisor Trap Cause
@@ -220,7 +220,7 @@ static inline uint64
 r_scause()
 {
   uint64 x;
-  asm("csrr %0, scause" : "=r" (x) );
+  asm volatile("csrr %0, scause" : "=r" (x) );
   return x;
 }
 
@@ -229,7 +229,7 @@ static inline uint64
 r_stval()
 {
   uint64 x;
-  asm("csrr %0, stval" : "=r" (x) );
+  asm volatile("csrr %0, stval" : "=r" (x) );
   return x;
 }
 
@@ -237,14 +237,14 @@ r_stval()
 static inline void 
 w_mcounteren(uint64 x)
 {
-  asm("csrw mcounteren, %0" : : "r" (x));
+  asm volatile("csrw mcounteren, %0" : : "r" (x));
 }
 
 static inline uint64
 r_mcounteren()
 {
   uint64 x;
-  asm("csrr %0, mcounteren" : "=r" (x) );
+  asm volatile("csrr %0, mcounteren" : "=r" (x) );
   return x;
 }
 
@@ -253,7 +253,7 @@ static inline uint64
 r_time()
 {
   uint64 x;
-  asm("csrr %0, time" : "=r" (x) );
+  asm volatile("csrr %0, time" : "=r" (x) );
   return x;
 }
 
@@ -284,7 +284,7 @@ static inline uint64
 r_sp()
 {
   uint64 x;
-  asm("mv %0, sp" : "=r" (x) );
+  asm volatile("mv %0, sp" : "=r" (x) );
   return x;
 }
 
@@ -294,14 +294,14 @@ static inline uint64
 r_tp()
 {
   uint64 x;
-  asm("mv %0, tp" : "=r" (x) );
+  asm volatile("mv %0, tp" : "=r" (x) );
   return x;
 }
 
 static inline void 
 w_tp(uint64 x)
 {
-  asm("mv tp, %0" : : "r" (x));
+  asm volatile("mv tp, %0" : : "r" (x));
 }
 
 #define PGSIZE 4096 // bytes per page
diff --git a/start.c b/start.c
index ce22be8..c4689dc 100644
--- a/start.c
+++ b/start.c
@@ -52,5 +52,5 @@ mstart()
   w_tp(id);
 
   // call main() in supervisor mode.
-  asm("mret");
+  asm volatile("mret");
 }