tcg/mips: Fix clobbering of qemu_ld inputs

The MIPS TCG backend implements qemu_ld with 64-bit targets using the v0
register (base) as a temporary to load the upper half of the QEMU TLB
comparator (see line 5 below), however this happens before the input
address is used (line 8 to mask off the low bits for the TLB
comparison, and line 12 to add the host-guest offset). If the input
address (addrl) also happens to have been placed in v0 (as in the second
column below), it gets clobbered before it is used.

addrl in t2 addrl in v0

1 srl a0,t2,0x7 srl a0,v0,0x7
2 andi a0,a0,0x1fe0 andi a0,a0,0x1fe0
3 addu a0,a0,s0 addu a0,a0,s0
4 lw at,9136(a0) lw at,9136(a0) set TCG_TMP0 (at)
5 lw v0,9140(a0) lw v0,9140(a0) set base (v0)
6 li t9,-4093 li t9,-4093
7 lw a0,9160(a0) lw a0,9160(a0) set addend (a0)
8 and t9,t9,t2 and t9,t9,v0 use addrl
9 bne at,t9,0x836d8c8 bne at,t9,0x836d838 use TCG_TMP0
10 nop nop
11 bne v0,t8,0x836d8c8 bne v0,a1,0x836d838 use base
12 addu v0,a0,t2 addu v0,a0,v0 use addrl, addend
13 lw t0,0(v0) lw t0,0(v0)

Fix by using TCG_TMP0 (at) as the temporary instead of v0 (base),
pushing the load on line 5 forward into the delay slot of the low
comparison (line 10). The early load of the addend on line 7 also needs
pushing even further for 64-bit targets, or it will clobber a0 before
we're done with it. The output for 32-bit targets is unaffected.

srl a0,v0,0x7
andi a0,a0,0x1fe0
addu a0,a0,s0
lw at,9136(a0)
-lw v0,9140(a0) load high comparator
li t9,-4093
-lw a0,9160(a0) load addend
and t9,t9,v0
bne at,t9,0x836d838
- nop
+ lw at,9140(a0) load high comparator
+lw a0,9160(a0) load addend
-bne v0,a1,0x836d838
+bne at,a1,0x836d838
addu v0,a0,v0
lw t0,0(v0)

Backports commit 33fca8589cf2aa7bf91564e6a8f26b3ba0910541 from qemu

qemu/tcg/mips/tcg-target.c

@@ -962,30 +962,34 @@ static void tcg_out_tlb_load(TCGContext *s, TCGReg base, TCGReg addrl,
         add_off -= 0x7ff0;
     }
 
-    /* Load the tlb comparator.  */
-    if (TARGET_LONG_BITS == 64) {
-        tcg_out_opc_imm(s, OPC_LW, TCG_TMP0, TCG_REG_A0, cmp_off + LO_OFF);
-        tcg_out_opc_imm(s, OPC_LW, base, TCG_REG_A0, cmp_off + HI_OFF);
-    } else {
-        tcg_out_opc_imm(s, OPC_LW, TCG_TMP0, TCG_REG_A0, cmp_off);
-    }
+    /* Load the (low half) tlb comparator.  */
+    tcg_out_opc_imm(s, OPC_LW, TCG_TMP0, TCG_REG_A0,
+                    cmp_off + (TARGET_LONG_BITS == 64 ? LO_OFF : 0));
 
     /* Mask the page bits, keeping the alignment bits to compare against.
-       In between, load the tlb addend for the fast path.  */
+       In between on 32-bit targets, load the tlb addend for the fast path.  */
     tcg_out_movi(s, TCG_TYPE_I32, TCG_TMP1,
                  TARGET_PAGE_MASK | ((1 << s_bits) - 1));
+    if (TARGET_LONG_BITS == 32) {
         tcg_out_opc_imm(s, OPC_LW, TCG_REG_A0, TCG_REG_A0, add_off);
+    }
     tcg_out_opc_reg(s, OPC_AND, TCG_TMP1, TCG_TMP1, addrl);
 
     label_ptr[0] = s->code_ptr;
     tcg_out_opc_br(s, OPC_BNE, TCG_TMP1, TCG_TMP0);
 
+    /* Load and test the high half tlb comparator.  */
     if (TARGET_LONG_BITS == 64) {
         /* delay slot */
-        tcg_out_nop(s);
+        tcg_out_opc_imm(s, OPC_LW, TCG_TMP0, TCG_REG_A0, cmp_off + HI_OFF);
+
+        /* Load the tlb addend for the fast path. We can't do it earlier with
+           64-bit targets or we'll clobber a0 before reading the high half tlb
+           comparator.  */
+        tcg_out_opc_imm(s, OPC_LW, TCG_REG_A0, TCG_REG_A0, add_off);
 
         label_ptr[1] = s->code_ptr;
-        tcg_out_opc_br(s, OPC_BNE, addrh, base);
+        tcg_out_opc_br(s, OPC_BNE, addrh, TCG_TMP0);
     }
 
     /* delay slot */