18.14 Anchored Addresses

GCC usually addresses every static object as a separate entity. For example, if we have:

static int a, b, c;
int foo (void) { return a + b + c; }

the code for foo will usually calculate three separate symbolic addresses: those of a, b and c. On some targets, it would be better to calculate just one symbolic address and access the three variables relative to it. The equivalent pseudocode would be something like:

int foo (void)
{
  register int *xr = &x;
  return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}

(which isn’t valid C). We refer to shared addresses like x as “section anchors”. Their use is controlled by -fsection-anchors.

The hooks below describe the target properties that GCC needs to know in order to make effective use of section anchors. It won’t use section anchors at all unless either TARGET_MIN_ANCHOR_OFFSET or TARGET_MAX_ANCHOR_OFFSET is set to a nonzero value.

Target Hook: HOST_WIDE_INT TARGET_MIN_ANCHOR_OFFSET

The minimum offset that should be applied to a section anchor. On most targets, it should be the smallest offset that can be applied to a base register while still giving a legitimate address for every mode. The default value is 0.

Target Hook: HOST_WIDE_INT TARGET_MAX_ANCHOR_OFFSET

Like TARGET_MIN_ANCHOR_OFFSET, but the maximum (inclusive) offset that should be applied to section anchors. The default value is 0.

Target Hook: void TARGET_ASM_OUTPUT_ANCHOR (rtx x)

Write the assembly code to define section anchor x, which is a SYMBOL_REF for which ‘SYMBOL_REF_ANCHOR_P (x)’ is true. The hook is called with the assembly output position set to the beginning of SYMBOL_REF_BLOCK (x).

If ASM_OUTPUT_DEF is available, the hook’s default definition uses it to define the symbol as ‘. + SYMBOL_REF_BLOCK_OFFSET (x)’. If ASM_OUTPUT_DEF is not available, the hook’s default definition is NULL, which disables the use of section anchors altogether.

Target Hook: bool TARGET_USE_ANCHORS_FOR_SYMBOL_P (const_rtx x)

Return true if GCC should attempt to use anchors to access SYMBOL_REF x. You can assume ‘SYMBOL_REF_HAS_BLOCK_INFO_P (x)’ and ‘!SYMBOL_REF_ANCHOR_P (x)’.

The default version is correct for most targets, but you might need to intercept this hook to handle things like target-specific attributes or target-specific sections.