From: Philipp Tomsich <philipp.tomsich@vrull.eu>
Date: Sat, 28 Jan 2023 23:07:09 +0000 (+0100)
Subject: aarch64: Correct the maximum shift amount for shifted operands
X-Git-Tag: basepoints/gcc-14~1725
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=2f2101c87ac88a9fa9f7b4a264fb7738118c7fc9;p=thirdparty%2Fgcc.git

aarch64: Correct the maximum shift amount for shifted operands

The aarch64 ISA specification allows a left shift amount to be applied
after extension in the range of 0 to 4 (encoded in the imm3 field).

This is true for at least the following instructions:

 * ADD (extend register)
 * ADDS (extended register)
 * SUB (extended register)

The result of this patch can be seen, when compiling the following code:

uint64_t myadd(uint64_t a, uint64_t b)
{
    return a+(((uint8_t)b)<<4);
}

Without the patch the following sequence will be generated:

0000000000000000 <myadd>:
   0:	d37c1c21 	ubfiz	x1, x1, #4, #8
   4:	8b000020 	add	x0, x1, x0
   8:	d65f03c0 	ret

With the patch the ubfiz will be merged into the add instruction:

0000000000000000 <myadd>:
   0:	8b211000 	add	x0, x0, w1, uxtb #4
   4:	d65f03c0 	ret

gcc/ChangeLog:

	* config/aarch64/aarch64.cc (aarch64_uxt_size): fix an
	off-by-one in checking the permissible shift-amount.
---

diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
index 089c1c858455..17c1e23e5b54 100644
--- a/gcc/config/aarch64/aarch64.cc
+++ b/gcc/config/aarch64/aarch64.cc
@@ -13022,7 +13022,7 @@ aarch64_output_casesi (rtx *operands)
 int
 aarch64_uxt_size (int shift, HOST_WIDE_INT mask)
 {
-  if (shift >= 0 && shift <= 3)
+  if (shift >= 0 && shift <= 4)
     {
       int size;
       for (size = 8; size <= 32; size *= 2)