target/ppc: Fix LQ, STQ register-pair order for big-endian

LQ, STQ have the same register-pair ordering as LQARX/STQARX., which is
the even (lower) register contains the most significant bits. This is
not implemented correctly for big-endian.

do_ldst_quad() has variables low_addr_gpr and high_addr_gpr which is
confusing because they are low and high addresses, whereas LQARX/STQARX.
and most such things use the low and high values for lo/hi variables.
The conversion to native 128-bit memory access functions missed this
strangeness.

Fix this by changing the if condition, and change the variable names to
hi/lo to match convention.

Cc: qemu-stable@nongnu.org
Reported-by: Ivan Warren <ivan@vmfacility.fr>
Fixes: 57b38ff ("target/ppc: Use tcg_gen_qemu_{ld,st}_i128 for LQARX, LQ, STQ")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1836
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

target/ppc/translate/fixedpoint-impl.c.inc

@@ -71,7 +71,7 @@ static bool do_ldst_quad(DisasContext *ctx, arg_D *a, bool store, bool prefixed)
 {
 #if defined(TARGET_PPC64)
     TCGv ea;
-    TCGv_i64 low_addr_gpr, high_addr_gpr;
+    TCGv_i64 lo, hi;
     TCGv_i128 t16;
 
     REQUIRE_INSNS_FLAGS(ctx, 64BX);
@@ -94,21 +94,21 @@ static bool do_ldst_quad(DisasContext *ctx, arg_D *a, bool store, bool prefixed)
     gen_set_access_type(ctx, ACCESS_INT);
     ea = do_ea_calc(ctx, a->ra, tcg_constant_tl(a->si));
 
-    if (prefixed || !ctx->le_mode) {
-        low_addr_gpr = cpu_gpr[a->rt];
-        high_addr_gpr = cpu_gpr[a->rt + 1];
+    if (ctx->le_mode && prefixed) {
+        lo = cpu_gpr[a->rt];
+        hi = cpu_gpr[a->rt + 1];
     } else {
-        low_addr_gpr = cpu_gpr[a->rt + 1];
-        high_addr_gpr = cpu_gpr[a->rt];
+        lo = cpu_gpr[a->rt + 1];
+        hi = cpu_gpr[a->rt];
     }
     t16 = tcg_temp_new_i128();
 
     if (store) {
-        tcg_gen_concat_i64_i128(t16, low_addr_gpr, high_addr_gpr);
+        tcg_gen_concat_i64_i128(t16, lo, hi);
         tcg_gen_qemu_st_i128(t16, ea, ctx->mem_idx, DEF_MEMOP(MO_128));
     } else {
         tcg_gen_qemu_ld_i128(t16, ea, ctx->mem_idx, DEF_MEMOP(MO_128));
-        tcg_gen_extr_i128_i64(low_addr_gpr, high_addr_gpr, t16);
+        tcg_gen_extr_i128_i64(lo, hi, t16);
     }
 #else
     qemu_build_not_reached();