[PATCH v2] target/arm: Merge arm_cpu_vq_map_next_smaller into sole caller

[Richard Henderson <richard.henderson@linaro.org>]

Coverity reports, in sve_zcr_get_valid_len,

"Subtract operation overflows on operands
arm_cpu_vq_map_next_smaller(cpu, start_vq + 1U) and 1U"

First, the aarch32 stub version of arm_cpu_vq_map_next_smaller,
returning 0, does exactly what Coverity reports.  Remove it.

Second, the aarch64 version of arm_cpu_vq_map_next_smaller has
a set of asserts, but they don't cover the case in question.
Further, there is a fair amount of extra arithmetic needed to
convert from the 0-based zcr register, to the 1-base vq form,
to the 0-based bitmap, and back again.  This can be simplified
by leaving the value in the 0-based form.

Finally, use test_bit to simplify the common case, where the
length in the zcr registers is in fact a supported length.

Reported-by: Coverity (CID 1407217)
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---

v2: Merge arm_cpu_vq_map_next_smaller into sve_zcr_get_valid_len,
    as suggested by Andrew Jones.

    Use test_bit to make the code even more obvious; the
    current_length + 1 thing to let us find current_length in the
    bitmap seemed unnecessarily clever.

---
 target/arm/cpu.h    |  3 ---
 target/arm/cpu64.c  | 15 ---------------
 target/arm/helper.c |  8 ++++++--
 3 files changed, 6 insertions(+), 20 deletions(-)

diff --git a/target/arm/cpu.h b/target/arm/cpu.h
index e1a66a2d1c..47d24a5375 100644
--- a/target/arm/cpu.h
+++ b/target/arm/cpu.h
@@ -185,12 +185,9 @@ typedef struct {
 #ifdef TARGET_AARCH64
 # define ARM_MAX_VQ    16
 void arm_cpu_sve_finalize(ARMCPU *cpu, Error **errp);
-uint32_t arm_cpu_vq_map_next_smaller(ARMCPU *cpu, uint32_t vq);
 #else
 # define ARM_MAX_VQ    1
 static inline void arm_cpu_sve_finalize(ARMCPU *cpu, Error **errp) { }
-static inline uint32_t arm_cpu_vq_map_next_smaller(ARMCPU *cpu, uint32_t vq)
-{ return 0; }
 #endif
 
 typedef struct ARMVectorReg {
diff --git a/target/arm/cpu64.c b/target/arm/cpu64.c
index 68baf0482f..a39d6fcea3 100644
--- a/target/arm/cpu64.c
+++ b/target/arm/cpu64.c
@@ -458,21 +458,6 @@ void arm_cpu_sve_finalize(ARMCPU *cpu, Error **errp)
     cpu->sve_max_vq = max_vq;
 }
 
-uint32_t arm_cpu_vq_map_next_smaller(ARMCPU *cpu, uint32_t vq)
-{
-    uint32_t bitnum;
-
-    /*
-     * We allow vq == ARM_MAX_VQ + 1 to be input because the caller may want
-     * to find the maximum vq enabled, which may be ARM_MAX_VQ, but this
-     * function always returns the next smaller than the input.
-     */
-    assert(vq && vq <= ARM_MAX_VQ + 1);
-
-    bitnum = find_last_bit(cpu->sve_vq_map, vq - 1);
-    return bitnum == vq - 1 ? 0 : bitnum + 1;
-}
-
 static void cpu_max_get_sve_max_vq(Object *obj, Visitor *v, const char *name,
                                    void *opaque, Error **errp)
 {
diff --git a/target/arm/helper.c b/target/arm/helper.c
index be67e2c66d..b5ee35a174 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -5363,9 +5363,13 @@ int sve_exception_el(CPUARMState *env, int el)
 
 static uint32_t sve_zcr_get_valid_len(ARMCPU *cpu, uint32_t start_len)
 {
-    uint32_t start_vq = (start_len & 0xf) + 1;
+    uint32_t end_len;
 
-    return arm_cpu_vq_map_next_smaller(cpu, start_vq + 1) - 1;
+    start_len &= 0xf;
+    end_len = find_last_bit(cpu->sve_vq_map, start_len + 1);
+
+    assert(end_len <= start_len);
+    return end_len;
 }
 
 /*
-- 
2.17.1