[RFC AArch64] Implement TARGET_PROMOTE_FUNCTION_MODE for ILP32 code generation

["Yangfei (Felix)" <felix dot yang at huawei dot com>]

Hi,

  For this trivial testcase: 

extern int bar (int , int);

int foo (int *a, int *b)
{
    return bar (*a, *b);
}

I noticed that GCC generate redundant zero-extension instructions under ILP32 (aarch64-linux-gnu-gcc -S -O2 -mabi=ilp32).
Assembly code:
        .arch armv8-a+fp+simd
        .file   "1.c"
        .text
        .align  2
        .p2align 3,,7
        .global foo
        .type   foo, %function
foo:
        uxtw    x0, w0
        uxtw    x1, w1
        ldr     w0, [x0]
        ldr     w1, [x1]
        b       bar
        .size   foo, .-foo
        .ident  "GCC: (20140403) 5.0.0 20150312 (experimental)"

According the ILP32 ABI, the two "uxtw" instrunctions here is not necessary.
The following is a proposed patch to fix this issue, i.e. promoting pointer function arguments to word_mode.
But I don't know whether it's a good idea to do this for pointer return values.
Any comments?


Index: gcc/config/aarch64/aarch64.c
===================================================================
--- gcc/config/aarch64/aarch64.c	(revision 221393)
+++ gcc/config/aarch64/aarch64.c	(working copy)
@@ -1587,7 +1587,7 @@ aarch64_function_value (const_tree type, const_tre
   machine_mode ag_mode;
 
   mode = TYPE_MODE (type);
-  if (INTEGRAL_TYPE_P (type))
+  if (INTEGRAL_TYPE_P (type) || POINTER_TYPE_P (type))
     mode = promote_function_mode (type, mode, &unsignedp, func, 1);
 
   if (aarch64_return_in_msb (type))
@@ -1650,6 +1650,24 @@ aarch64_function_value_regno_p (const unsigned int
   return false;
 }
 
+/* Implement TARGET_PROMOTE_FUNCTION_MODE.  */
+
+static machine_mode
+aarch64_promote_function_mode (const_tree type, machine_mode mode,
+			       int *punsignedp, const_tree fntype,
+			       int for_return)
+{
+  /* Pointer function arguments and return values are promoted to word_mode.  */
+  if (type != NULL_TREE && POINTER_TYPE_P (type))
+    {
+      *punsignedp = POINTERS_EXTEND_UNSIGNED;
+      return word_mode;
+    }
+
+  return default_promote_function_mode (type, mode, punsignedp, fntype,
+                                        for_return);
+}
+
 /* Implement TARGET_RETURN_IN_MEMORY.
 
    If the type T of the result of a function is such that
@@ -11329,6 +11347,9 @@ aarch64_gen_adjusted_ldpstp (rtx *operands, bool l
 #define TARGET_OVERRIDE_OPTIONS_AFTER_CHANGE \
   aarch64_override_options_after_change
 
+#undef TARGET_PROMOTE_FUNCTION_MODE
+#define TARGET_PROMOTE_FUNCTION_MODE aarch64_promote_function_mode
+
 #undef TARGET_PASS_BY_REFERENCE
 #define TARGET_PASS_BY_REFERENCE aarch64_pass_by_reference

Attachment: aarch64-promote-v2.diff
Description: aarch64-promote-v2.diff