diff --git a/target/riscv/bitmanip_helper.c b/target/riscv/bitmanip_helper.c
index bb48388fcd..f1b5e5549f 100644
--- a/target/riscv/bitmanip_helper.c
+++ b/target/riscv/bitmanip_helper.c
@@ -24,46 +24,6 @@
 #include "exec/helper-proto.h"
 #include "tcg/tcg.h"
 
-static const uint64_t adjacent_masks[] = {
-    dup_const(MO_8, 0x55),
-    dup_const(MO_8, 0x33),
-    dup_const(MO_8, 0x0f),
-    dup_const(MO_16, 0xff),
-    dup_const(MO_32, 0xffff),
-    UINT32_MAX
-};
-
-static inline target_ulong do_swap(target_ulong x, uint64_t mask, int shift)
-{
-    return ((x & mask) << shift) | ((x & ~mask) >> shift);
-}
-
-static target_ulong do_grev(target_ulong rs1,
-                            target_ulong rs2,
-                            int bits)
-{
-    target_ulong x = rs1;
-    int i, shift;
-
-    for (i = 0, shift = 1; shift < bits; i++, shift <<= 1) {
-        if (rs2 & shift) {
-            x = do_swap(x, adjacent_masks[i], shift);
-        }
-    }
-
-    return x;
-}
-
-target_ulong HELPER(grev)(target_ulong rs1, target_ulong rs2)
-{
-    return do_grev(rs1, rs2, TARGET_LONG_BITS);
-}
-
-target_ulong HELPER(grevw)(target_ulong rs1, target_ulong rs2)
-{
-    return do_grev(rs1, rs2, 32);
-}
-
 target_ulong HELPER(clmul)(target_ulong rs1, target_ulong rs2)
 {
     target_ulong result = 0;
diff --git a/target/riscv/helper.h b/target/riscv/helper.h
index a9bda2c8ac..c7a5376227 100644
--- a/target/riscv/helper.h
+++ b/target/riscv/helper.h
@@ -59,8 +59,6 @@ DEF_HELPER_FLAGS_2(fcvt_d_lu, TCG_CALL_NO_RWG, i64, env, tl)
 DEF_HELPER_FLAGS_1(fclass_d, TCG_CALL_NO_RWG_SE, tl, i64)
 
 /* Bitmanip */
-DEF_HELPER_FLAGS_2(grev, TCG_CALL_NO_RWG_SE, tl, tl, tl)
-DEF_HELPER_FLAGS_2(grevw, TCG_CALL_NO_RWG_SE, tl, tl, tl)
 DEF_HELPER_FLAGS_2(clmul, TCG_CALL_NO_RWG_SE, tl, tl, tl)
 DEF_HELPER_FLAGS_2(clmulr, TCG_CALL_NO_RWG_SE, tl, tl, tl)
 
diff --git a/target/riscv/insn32.decode b/target/riscv/insn32.decode
index 59202196dc..901a66c0f5 100644
--- a/target/riscv/insn32.decode
+++ b/target/riscv/insn32.decode
@@ -683,6 +683,9 @@ min        0000101 .......... 100 ..... 0110011 @r
 minu       0000101 .......... 101 ..... 0110011 @r
 orc_b      001010 000111 ..... 101 ..... 0010011 @r2
 orn        0100000 .......... 110 ..... 0110011 @r
+# The encoding for rev8 differs between RV32 and RV64.
+# rev8_32 denotes the RV32 variant.
+rev8_32    011010 011000 ..... 101 ..... 0010011 @r2
 rol        0110000 .......... 001 ..... 0110011 @r
 ror        0110000 .......... 101 ..... 0110011 @r
 rori       01100 ............ 101 ..... 0010011 @sh
@@ -694,6 +697,10 @@ xnor       0100000 .......... 100 ..... 0110011 @r
 clzw       0110000 00000 ..... 001 ..... 0011011 @r2
 ctzw       0110000 00001 ..... 001 ..... 0011011 @r2
 cpopw      0110000 00010 ..... 001 ..... 0011011 @r2
+# The encoding for rev8 differs between RV32 and RV64.
+# When executing on RV64, the encoding used in RV32 is an illegal
+# instruction, so we use different handler functions to differentiate.
+rev8_64    011010 111000 ..... 101 ..... 0010011 @r2
 rolw       0110000 .......... 001 ..... 0111011 @r
 roriw      0110000 .......... 101 ..... 0011011 @sh5
 rorw       0110000 .......... 101 ..... 0111011 @r
@@ -702,15 +709,10 @@ rorw       0110000 .......... 101 ..... 0111011 @r
 pack       0000100 .......... 100 ..... 0110011 @r
 packu      0100100 .......... 100 ..... 0110011 @r
 packh      0000100 .......... 111 ..... 0110011 @r
-grev       0110100 .......... 101 ..... 0110011 @r
-grevi      01101. ........... 101 ..... 0010011 @sh
 
 # *** RV64B Standard Extension (in addition to RV32B) ***
 packw      0000100 .......... 100 ..... 0111011 @r
 packuw     0100100 .......... 100 ..... 0111011 @r
-grevw      0110100 .......... 101 ..... 0111011 @r
-
-greviw     0110100 .......... 101 ..... 0011011 @sh5
 
 # *** RV32 Zbc Standard Extension ***
 clmul      0000101 .......... 001 ..... 0110011 @r
diff --git a/target/riscv/insn_trans/trans_rvb.c.inc b/target/riscv/insn_trans/trans_rvb.c.inc
index d32af5915a..48a7c9ca5e 100644
--- a/target/riscv/insn_trans/trans_rvb.c.inc
+++ b/target/riscv/insn_trans/trans_rvb.c.inc
@@ -273,26 +273,18 @@ static bool trans_rol(DisasContext *ctx, arg_rol *a)
     return gen_shift(ctx, a, EXT_NONE, tcg_gen_rotl_tl);
 }
 
-static bool trans_grev(DisasContext *ctx, arg_grev *a)
+static bool trans_rev8_32(DisasContext *ctx, arg_rev8_32 *a)
 {
-    REQUIRE_EXT(ctx, RVB);
-    return gen_shift(ctx, a, EXT_NONE, gen_helper_grev);
+    REQUIRE_32BIT(ctx);
+    REQUIRE_ZBB(ctx);
+    return gen_unary(ctx, a, EXT_NONE, tcg_gen_bswap_tl);
 }
 
-static void gen_grevi(TCGv dest, TCGv src, target_long shamt)
+static bool trans_rev8_64(DisasContext *ctx, arg_rev8_64 *a)
 {
-    if (shamt == TARGET_LONG_BITS - 8) {
-        /* rev8, byte swaps */
-        tcg_gen_bswap_tl(dest, src);
-    } else {
-        gen_helper_grev(dest, src, tcg_constant_tl(shamt));
-    }
-}
-
-static bool trans_grevi(DisasContext *ctx, arg_grevi *a)
-{
-    REQUIRE_EXT(ctx, RVB);
-    return gen_shift_imm_fn(ctx, a, EXT_NONE, gen_grevi);
+    REQUIRE_64BIT(ctx);
+    REQUIRE_ZBB(ctx);
+    return gen_unary(ctx, a, EXT_NONE, tcg_gen_bswap_tl);
 }
 
 static void gen_orc_b(TCGv ret, TCGv source1)
@@ -471,22 +463,6 @@ static bool trans_rolw(DisasContext *ctx, arg_rolw *a)
     return gen_shift(ctx, a, EXT_NONE, gen_rolw);
 }
 
-static bool trans_grevw(DisasContext *ctx, arg_grevw *a)
-{
-    REQUIRE_64BIT(ctx);
-    REQUIRE_EXT(ctx, RVB);
-    ctx->w = true;
-    return gen_shift(ctx, a, EXT_ZERO, gen_helper_grev);
-}
-
-static bool trans_greviw(DisasContext *ctx, arg_greviw *a)
-{
-    REQUIRE_64BIT(ctx);
-    REQUIRE_EXT(ctx, RVB);
-    ctx->w = true;
-    return gen_shift_imm_tl(ctx, a, EXT_ZERO, gen_helper_grev);
-}
-
 #define GEN_SHADD_UW(SHAMT)                                       \
 static void gen_sh##SHAMT##add_uw(TCGv ret, TCGv arg1, TCGv arg2) \
 {                                                                 \