[Qemu-devel] [4548] Comment non-obvious calculation.

[Andrzej Zaborowski]

Revision: 4548
          http://svn.sv.gnu.org/viewvc/?view=rev&root=qemu&revision=4548
Author:   balrog
Date:     2008-05-23 18:51:15 +0000 (Fri, 23 May 2008)

Log Message:
-----------
Comment non-obvious calculation.  Don't clobber r3 in qemu_st64.

Modified Paths:
--------------
    trunk/tcg/arm/tcg-target.c

Modified: trunk/tcg/arm/tcg-target.c
===================================================================
--- trunk/tcg/arm/tcg-target.c  2008-05-23 18:50:44 UTC (rev 4547)
+++ trunk/tcg/arm/tcg-target.c  2008-05-23 18:51:15 UTC (rev 4548)
@@ -854,12 +854,27 @@
     s_bits = opc & 3;
 
 # ifdef USE_TLB
+    /* Should generate something like the following:
+     *  ror r8, addr_reg, #TARGET_PAGE_BITS
+     *  and r0, r8, #(CPU_TLB_SIZE - 1)   @ Assumption: CPU_TLB_BITS <= 8
+     *  add r0, T0, r0 lsl #CPU_TLB_ENTRY_BITS
+     */
+#  if CPU_TLB_BITS > 8
+#   error
+#  endif
     tcg_out_dat_reg(s, COND_AL, ARITH_MOV,
                     8, 0, addr_reg, SHIFT_IMM_ROR(TARGET_PAGE_BITS));
     tcg_out_dat_imm(s, COND_AL, ARITH_AND,
                     0, 8, CPU_TLB_SIZE - 1);
     tcg_out_dat_reg(s, COND_AL, ARITH_ADD,
                     0, TCG_AREG0, 0, SHIFT_IMM_LSL(CPU_TLB_ENTRY_BITS));
+    /* In the
+     *  ldr r1 [r0, #(offsetof(CPUState, tlb_table[mem_index][0].addr_read))]
+     * below, the offset is likely to exceed 12 bits if mem_index != 0 and
+     * not exceed otherwise, so use an
+     *  add r0, r0, #(mem_index * sizeof *CPUState.tlb_table)
+     * before.
+     */
 #  define TLB_SHIFT    (CPU_TLB_ENTRY_BITS + CPU_TLB_BITS)
     if (mem_index)
         tcg_out_dat_imm(s, COND_AL, ARITH_ADD, 0, 0,
@@ -1025,12 +1040,24 @@
     s_bits = opc & 3;
 
 # ifdef USE_TLB
+    /* Should generate something like the following:
+     *  ror r8, addr_reg, #TARGET_PAGE_BITS
+     *  and r0, r8, #(CPU_TLB_SIZE - 1)   @ Assumption: CPU_TLB_BITS <= 8
+     *  add r0, T0, r0 lsl #CPU_TLB_ENTRY_BITS
+     */
     tcg_out_dat_reg(s, COND_AL, ARITH_MOV,
                     8, 0, addr_reg, SHIFT_IMM_ROR(TARGET_PAGE_BITS));
     tcg_out_dat_imm(s, COND_AL, ARITH_AND,
                     0, 8, CPU_TLB_SIZE - 1);
     tcg_out_dat_reg(s, COND_AL, ARITH_ADD,
                     0, TCG_AREG0, 0, SHIFT_IMM_LSL(CPU_TLB_ENTRY_BITS));
+    /* In the
+     *  ldr r1 [r0, #(offsetof(CPUState, tlb_table[mem_index][0].addr_write))]
+     * below, the offset is likely to exceed 12 bits if mem_index != 0 and
+     * not exceed otherwise, so use an
+     *  add r0, r0, #(mem_index * sizeof *CPUState.tlb_table)
+     * before.
+     */
     if (mem_index)
         tcg_out_dat_imm(s, COND_AL, ARITH_ADD, 0, 0,
                         (mem_index << (TLB_SHIFT & 1)) |
@@ -1084,10 +1111,6 @@
     tcg_out_b(s, COND_EQ, 8);
 # endif
 
-# ifdef SAVE_LR
-    tcg_out_dat_reg(s, cond, ARITH_MOV, 8, 0, 14, SHIFT_IMM_LSL(0));
-# endif
-
     /* TODO: move this code to where the constants pool will be */
     if (addr_reg)
         tcg_out_dat_reg(s, cond, ARITH_MOV,
@@ -1144,8 +1167,8 @@
         tcg_out_dat_imm(s, cond, ARITH_MOV, 3, 0, mem_index);
         break;
     case 3:
-        tcg_out_dat_imm(s, cond, ARITH_MOV, 3, 0, mem_index);
-        tcg_out32(s, (cond << 28) | 0x052d3010); /* str r3, [sp, #-0x10]! */
+        tcg_out_dat_imm(s, cond, ARITH_MOV, 8, 0, mem_index);
+        tcg_out32(s, (cond << 28) | 0x052d8010); /* str r8, [sp, #-0x10]! */
         if (data_reg != 2)
             tcg_out_dat_reg(s, cond, ARITH_MOV,
                             2, 0, data_reg, SHIFT_IMM_LSL(0));
@@ -1156,6 +1179,10 @@
     }
 # endif
 
+# ifdef SAVE_LR
+    tcg_out_dat_reg(s, cond, ARITH_MOV, 8, 0, 14, SHIFT_IMM_LSL(0));
+# endif
+
     tcg_out_bl(s, cond, (tcg_target_long) qemu_st_helpers[s_bits] -
                     (tcg_target_long) s->code_ptr);