[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, #1
        str     r1, [r2]
        subw    r2, r3, #2031
        movs    r1, #2
        str     r1, [r2]
        subw    r2, r3, #2043
        movs    r1, #3
        str     r1, [r2]
        subw    r2, r3, #2019
        movs    r1, #4
        subw    r3, r3, #2047
        str     r1, [r2]
        movs    r2, #6
        str     r2, [r3]
        bx      lr

We now get

        ldr     r3, .L2
        movs    r2, #1
        str     r2, [r3, #2060]
        movs    r2, #2
        str     r2, [r3, #2064]
        movs    r2, #3
        str     r2, [r3, #2052]
        movs    r2, #4
        str     r2, [r3, #2076]
        movs    r2, #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

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index e520cf054036f875eedc22e074a0d8ee96448fa5..66b7a1422519afa53eb08da7a84efe5b67ce6d94 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,8 @@
+2019-10-31  Richard Earnshaw  <rearnsha@arm.com>
+
+       * 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  <rearnsha@arm.com>
 
        * config/arm/arm.md (add_not_cin): New insn.

diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 3c83b7f7f69ba6cecbdb26c82d3893b92e0683e4..eddd3ca93ed62915bee3a3f5a396f0be069bd31f 100644 (file)
--- a/gcc/config/arm/arm.c
+++ b/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;
        }