[PATCH 2/2] target/arm: Implement M-profile trapping on division by zero

[Peter Maydell <peter.maydell@linaro.org>]

Unlike A-profile, for M-profile the UDIV and SDIV insns can be
configured to raise an exception on division by zero, using the CCR
DIV_0_TRP bit.

Implement support for setting this bit by making the helper functions
raise the appropriate exception.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 target/arm/cpu.h       |  1 +
 target/arm/helper.h    |  4 ++--
 target/arm/helper.c    | 19 +++++++++++++++++--
 target/arm/m_helper.c  |  4 ++++
 target/arm/translate.c |  4 ++--
 5 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/target/arm/cpu.h b/target/arm/cpu.h
index 9f0a5f84d50..5cf8996ae3c 100644
--- a/target/arm/cpu.h
+++ b/target/arm/cpu.h
@@ -54,6 +54,7 @@
 #define EXCP_LAZYFP         20   /* v7M fault during lazy FP stacking */
 #define EXCP_LSERR          21   /* v8M LSERR SecureFault */
 #define EXCP_UNALIGNED      22   /* v7M UNALIGNED UsageFault */
+#define EXCP_DIVBYZERO      23   /* v7M DIVBYZERO UsageFault */
 /* NB: add new EXCP_ defines to the array in arm_log_exception() too */
 
 #define ARMV7M_EXCP_RESET   1
diff --git a/target/arm/helper.h b/target/arm/helper.h
index 248569b0cd8..aee8f0019b4 100644
--- a/target/arm/helper.h
+++ b/target/arm/helper.h
@@ -6,8 +6,8 @@ DEF_HELPER_3(add_saturate, i32, env, i32, i32)
 DEF_HELPER_3(sub_saturate, i32, env, i32, i32)
 DEF_HELPER_3(add_usaturate, i32, env, i32, i32)
 DEF_HELPER_3(sub_usaturate, i32, env, i32, i32)
-DEF_HELPER_FLAGS_2(sdiv, TCG_CALL_NO_RWG_SE, s32, s32, s32)
-DEF_HELPER_FLAGS_2(udiv, TCG_CALL_NO_RWG_SE, i32, i32, i32)
+DEF_HELPER_FLAGS_3(sdiv, TCG_CALL_NO_RWG, s32, env, s32, s32)
+DEF_HELPER_FLAGS_3(udiv, TCG_CALL_NO_RWG, i32, env, i32, i32)
 DEF_HELPER_FLAGS_1(rbit, TCG_CALL_NO_RWG_SE, i32, i32)
 
 #define PAS_OP(pfx)  \
diff --git a/target/arm/helper.c b/target/arm/helper.c
index 8e9c2a2cf8c..56c520cf8e9 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -9345,6 +9345,18 @@ uint32_t HELPER(sxtb16)(uint32_t x)
     return res;
 }
 
+static void handle_possible_div0_trap(CPUARMState *env, uintptr_t ra)
+{
+    /*
+     * Take a division-by-zero exception if necessary; otherwise return
+     * to get the usual non-trapping division behaviour (result of 0)
+     */
+    if (arm_feature(env, ARM_FEATURE_M)
+        && (env->v7m.ccr[env->v7m.secure] & R_V7M_CCR_DIV_0_TRP_MASK)) {
+        raise_exception_ra(env, EXCP_DIVBYZERO, 0, 1, ra);
+    }
+}
+
 uint32_t HELPER(uxtb16)(uint32_t x)
 {
     uint32_t res;
@@ -9353,9 +9365,10 @@ uint32_t HELPER(uxtb16)(uint32_t x)
     return res;
 }
 
-int32_t HELPER(sdiv)(int32_t num, int32_t den)
+int32_t HELPER(sdiv)(CPUARMState *env, int32_t num, int32_t den)
 {
     if (den == 0) {
+        handle_possible_div0_trap(env, GETPC());
         return 0;
     }
     if (num == INT_MIN && den == -1) {
@@ -9364,9 +9377,10 @@ int32_t HELPER(sdiv)(int32_t num, int32_t den)
     return num / den;
 }
 
-uint32_t HELPER(udiv)(uint32_t num, uint32_t den)
+uint32_t HELPER(udiv)(CPUARMState *env, uint32_t num, uint32_t den)
 {
     if (den == 0) {
+        handle_possible_div0_trap(env, GETPC());
         return 0;
     }
     return num / den;
@@ -9567,6 +9581,7 @@ void arm_log_exception(int idx)
             [EXCP_LAZYFP] = "v7M exception during lazy FP stacking",
             [EXCP_LSERR] = "v8M LSERR UsageFault",
             [EXCP_UNALIGNED] = "v7M UNALIGNED UsageFault",
+            [EXCP_DIVBYZERO] = "v7M DIVBYZERO UsageFault",
         };
 
         if (idx >= 0 && idx < ARRAY_SIZE(excnames)) {
diff --git a/target/arm/m_helper.c b/target/arm/m_helper.c
index 20761c94877..47903b3dc35 100644
--- a/target/arm/m_helper.c
+++ b/target/arm/m_helper.c
@@ -2252,6 +2252,10 @@ void arm_v7m_cpu_do_interrupt(CPUState *cs)
         armv7m_nvic_set_pending(env->nvic, ARMV7M_EXCP_USAGE, env->v7m.secure);
         env->v7m.cfsr[env->v7m.secure] |= R_V7M_CFSR_UNALIGNED_MASK;
         break;
+    case EXCP_DIVBYZERO:
+        armv7m_nvic_set_pending(env->nvic, ARMV7M_EXCP_USAGE, env->v7m.secure);
+        env->v7m.cfsr[env->v7m.secure] |= R_V7M_CFSR_DIVBYZERO_MASK;
+        break;
     case EXCP_SWI:
         /* The PC already points to the next instruction.  */
         armv7m_nvic_set_pending(env->nvic, ARMV7M_EXCP_SVC, env->v7m.secure);
diff --git a/target/arm/translate.c b/target/arm/translate.c
index 80c282669f0..28eabeb2323 100644
--- a/target/arm/translate.c
+++ b/target/arm/translate.c
@@ -7992,9 +7992,9 @@ static bool op_div(DisasContext *s, arg_rrr *a, bool u)
     t1 = load_reg(s, a->rn);
     t2 = load_reg(s, a->rm);
     if (u) {
-        gen_helper_udiv(t1, t1, t2);
+        gen_helper_udiv(t1, cpu_env, t1, t2);
     } else {
-        gen_helper_sdiv(t1, t1, t2);
+        gen_helper_sdiv(t1, cpu_env, t1, t2);
     }
     tcg_temp_free_i32(t2);
     store_reg(s, a->rd, t1);
-- 
2.20.1