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xtensa: Apply further improvement to xtensa_legitimize_address()

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The load/store instructions in the Xtensa ISA have an unsigned 8-bit
displacement immediate field that scales with the byte width of the
reference.  That is, for a 1-byte reference, the displacement is between
0 and 255, for 2-bytes between 0 and 510, and for 4-bytes between 0 and
1020.

However, xtensa_legitimize_address() has not been able to take advantage
of this fact until now, and has limited the maximum displacement to 255
regardless of the reference byte width.

This patch resolves the above limitation and slightly improves the effi-
ciency of large positive displacements during memory accesses wider than
1-byte.

     /* example */
     int test(short a[]) {
       return a[32767] + a[16511] + a[1];
     }

     ;; before (-O2)
     	.literal_position
     	.literal .LC0, 65534
     test:
     	entry	sp, 32
     	l32r	a8, .LC0
     	addmi	a9, a2, 0x100
     	add.n	a8, a2, a8
     	addmi	a9, a9, 0x7f00
     	l16si	a8, a8, 0	;; 32767 = 65534 / 2
     	l16si	a9, a9, 254	;; 16551 = (32512 + 256 + 254) / 2
     	l16si	a2, a2, 2
     	add.n	a8, a8, a9
     	add.n	a2, a8, a2
     	retw.n

     ;; after (-O2)
     test:
     	entry	sp, 32
     	addmi	a9, a2, 0x7f00	;; CSEd
     	addmi	a8, a9, 0x7f00
     	l16si	a8, a8, 510	;; 32767 = (32512 + 32512 + 510) / 2
     	l16si	a9, a9, 510	;; 16511 = (32512 + 510) / 2
     	l16si	a2, a2, 2
     	add.n	a8, a8, a9
     	add.n	a2, a8, a2
     	retw.n

gcc/ChangeLog:

	* config/xtensa/xtensa.cc (xtensa_legitimize_address):
	Modify to extend the upper limit of the coverable offset if the
	address displacement of the corresponding machine instruction is
	greater than 255.
This commit is contained in:
Takayuki 'January June' Suwa
2026-05-05 08:55:15 +09:00
committed by Max Filippov
parent 234d9acfd2
commit bba0342a57
1 changed files with 50 additions and 25 deletions
Download Patch File Download Diff File Expand all files Collapse all files

75
gcc/config/xtensa/xtensa.cc
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@@ -2340,8 +2340,9 @@ xtensa_legitimize_address (rtx x,
rtx oldx ATTRIBUTE_UNUSED,
machine_mode mode)
{
rtx plus0, plus1, temp;
HOST_WIDE_INT offset, mem_offset, addmi_offset;
rtx plus0, plus1, temp0, temp1;
HOST_WIDE_INT offset, mem_disp, delta, offset2;
int mode_size;
if (xtensa_tls_symbol_p (x))
return xtensa_legitimize_tls_address (x);
@@ -2353,33 +2354,57 @@ xtensa_legitimize_address (rtx x,
if (! REG_P (plus0) && REG_P (plus1))
std::swap (plus0, plus1);
/* Try to split up the offset to use up to two ADDMI instructions. */
if (REG_P (plus0) && CONST_INT_P (plus1)
&& ! xtensa_mem_offset (offset = INTVAL (plus1), mode)
&& ! xtensa_simm8 (offset)
&& xtensa_mem_offset (mem_offset = offset & 0xff, mode))
/* Try to split up the offset to use up to two ADDMI instructions;
The two ADDMIs are slightly more efficient than "L32R w/litpool + ADD"
or "CONST16 pair + ADD", if applicable. */
if (! REG_P (plus0) || ! CONST_INT_P (plus1)
|| xtensa_mem_offset (offset = INTVAL (plus1), mode)
|| xtensa_simm8 (offset)
|| ! xtensa_mem_offset (mem_disp = offset & 0xff, mode))
return x;
/* The above assumes that the displacement within the load/store instruc-
tion is unsigned 8 bits, regardless of the load/store width. However,
in actual 2- or 4-byte width load/store instructions, a displacement
shifted by 1 or 2 bits, respectively, is added to the base register.
Here, determine the amount of displacement delta that these instructions
can cover extra range. */
delta = (mode_size = GET_MODE_SIZE (mode)) >= 4 ? 768 :
mode_size == 2 ? 256 : 0;
/* The upper limit of the ADDMI instruction's addition is allowed to be
widened by the delta amount calculated above, and the excess is later
renormalized to the displacement of the load/store instrution. */
offset2 = offset & ~0xff, offset = 0;
if (! IN_RANGE (offset2, -32768, 32512 + delta))
{
/* The two ADDMIs are slightly more efficient than
"L32R w/litpool + ADD" or "CONST16 pair + ADD", if applicable. */
addmi_offset = offset & ~0xff;
if (addmi_offset > 32512)
offset = 32512, addmi_offset -= 32512;
else if (addmi_offset < -32768)
offset = -32768, addmi_offset += 32768;
else
offset = 0;
if (offset2 > 32512)
offset = 32512, offset2 -= 32512;
else if (offset2 < -32768)
offset = -32768, offset2 += 32768;
if (xtensa_simm8x256 (addmi_offset))
{
emit_insn (gen_addsi3 (temp = gen_reg_rtx (Pmode),
plus0, GEN_INT (addmi_offset)));
if (offset)
emit_insn (gen_addsi3 (temp, temp, GEN_INT (offset)));
return gen_rtx_PLUS (Pmode, temp, GEN_INT (mem_offset));
}
/* If two ADDMIs are not enough, the process will be canceled. */
if (! IN_RANGE (offset2, -32768, 32512 + delta))
return x;
}
if (offset2 > 32512)
mem_disp += offset2 - 32512, offset2 = 32512;
return x;
/* Emit one or two ADDMI instructions, and then return an address RTX
with the remaining offset.
By adding the offset with the largest absolute value first via
temporary pseudos, the likelihood of those pseudos being consolidated
by the CSE increases. */
temp0 = gen_reg_rtx (Pmode);
if (offset)
{
emit_insn (gen_addsi3 (temp1 = gen_reg_rtx (Pmode),
plus0, GEN_INT (offset)));
emit_insn (gen_addsi3 (temp0, temp1, GEN_INT (offset2)));
}
else
emit_insn (gen_addsi3 (temp0, plus0, GEN_INT (offset2)));
return gen_rtx_PLUS (Pmode, temp0, GEN_INT (mem_disp));
}
/* Worker function for TARGET_MODE_DEPENDENT_ADDRESS_P.