[arm] Avoid using negative offsets for 'immediate' addresses when compiling for Thumb2

Thumb2 code now uses the Arm implementation of legitimize_address.
That code has a case to handle addresses that are absolute CONST_INT
values, which is a common use case in deeply embedded targets (eg:
void *p = (void*)0x12345678).  Since thumb has very limited negative
offsets from a constant, we want to avoid forming a CSE base that will
then be used with a negative value.

This was reported upstream originally in
https://gcc.gnu.org/ml/gcc-help/2019-10/msg00122.html

For example,

void test1(void) {
  volatile uint32_t * const p = (uint32_t *) 0x43fe1800;

  p[3] = 1;
  p[4] = 2;
  p[1] = 3;
  p[7] = 4;
  p[0] = 6;
}

With the new code, instead of 

        ldr     r3, .L2
        subw    r2, r3, #2035
        movs    r1, gcc-mirror#1
        str     r1, [r2]
        subw    r2, r3, #2031
        movs    r1, gcc-mirror#2
        str     r1, [r2]
        subw    r2, r3, #2043
        movs    r1, gcc-mirror#3
        str     r1, [r2]
        subw    r2, r3, #2019
        movs    r1, gcc-mirror#4
        subw    r3, r3, #2047
        str     r1, [r2]
        movs    r2, gcc-mirror#6
        str     r2, [r3]
        bx      lr


We now get

        ldr     r3, .L2
        movs    r2, gcc-mirror#1
        str     r2, [r3, #2060]
        movs    r2, gcc-mirror#2
        str     r2, [r3, #2064]
        movs    r2, gcc-mirror#3
        str     r2, [r3, #2052]
        movs    r2, gcc-mirror#4
        str     r2, [r3, #2076]
        movs    r2, gcc-mirror#6
        str     r2, [r3, #2048]
        bx      lr


	* config/arm/arm.c (arm_legitimize_address): Don't form negative
	offsets from a CONST_INT address when TARGET_THUMB2.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@277677 138bc75d-0d04-0410-961f-82ee72b054a4

Author: rearnsha

gcc/ChangeLog

@@ -1,3 +1,8 @@
+2019-10-31  Richard Earnshaw  <[email protected]>
+
+	* config/arm/arm.c (arm_legitimize_address): Don't form negative offsets
+	from a CONST_INT address when TARGET_THUMB2.
+
 2019-10-31  Richard Earnshaw  <[email protected]>
 
 	* config/arm/arm.md (add_not_cin): New insn.

gcc/config/arm/arm.c

@@ -9039,17 +9039,20 @@ arm_legitimize_address (rtx x, rtx orig_x, machine_mode mode)
       HOST_WIDE_INT mask, base, index;
       rtx base_reg;
 
-      /* ldr and ldrb can use a 12-bit index, ldrsb and the rest can only
-         use a 8-bit index. So let's use a 12-bit index for SImode only and
-         hope that arm_gen_constant will enable ldrb to use more bits. */
+      /* LDR and LDRB can use a 12-bit index, ldrsb and the rest can
+	 only use a 8-bit index. So let's use a 12-bit index for
+	 SImode only and hope that arm_gen_constant will enable LDRB
+	 to use more bits. */
       bits = (mode == SImode) ? 12 : 8;
       mask = (1 << bits) - 1;
       base = INTVAL (x) & ~mask;
       index = INTVAL (x) & mask;
-      if (bit_count (base & 0xffffffff) > (32 - bits)/2)
-        {
-	  /* It'll most probably be more efficient to generate the base
-	     with more bits set and use a negative index instead. */
+      if (TARGET_ARM && bit_count (base & 0xffffffff) > (32 - bits)/2)
+	{
+	  /* It'll most probably be more efficient to generate the
+	     base with more bits set and use a negative index instead.
+	     Don't do this for Thumb as negative offsets are much more
+	     limited.  */
 	  base |= mask;
 	  index -= mask;
 	}