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aarch64: eliminate redundant zero extend after bitwise negation

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The attached patch eliminates a redundant zero extend from the AArch64 backend. Given the following C code:

unsigned long long foo(unsigned a)
{
    return ~a;
}

prior to this patch, AArch64 GCC at -O2 generates:

foo:
        mvn     w0, w0
        uxtw    x0, w0
        ret

but the uxtw is redundant, since the mvn clears the upper half of the x0 register. After applying this patch, GCC at -O2 gives:

foo:
        mvn     w0, w0
        ret

Testing:
    Added regression test which passes after applying the change to aarch64.md.
    Full bootstrap and regression on aarch64-linux with no additional failures.

        * config/aarch64/aarch64.md (*one_cmpl_zero_extend): New.
        * gcc.target/aarch64/mvn_zero_ext.c: New test.
This commit is contained in:
Alex Coplan 2020-05-05 10:33:02 +01:00 committed by Kyrylo Tkachov
parent 144aee70b8
commit 1bd3a8af85
4 changed files with 32 additions and 0 deletions
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4
gcc/ChangeLog
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@ -1,3 +1,7 @@
2020-05-05 Alex Coplan <alex.coplan@arm.com>
* config/aarch64/aarch64.md (*one_cmpl_zero_extend): New.
2020-05-05 Jakub Jelinek <jakub@redhat.com>
PR tree-optimization/94800

9
gcc/config/aarch64/aarch64.md
View File

@ -4619,6 +4619,15 @@
(set_attr "arch" "*,simd")]
)
(define_insn "*one_cmpl_zero_extend"
[(set (match_operand:DI 0 "register_operand" "=r")
(zero_extend:DI
(not:SI (match_operand:SI 1 "register_operand" "r"))))]
""
"mvn\\t%w0, %w1"
[(set_attr "type" "logic_reg")]
)
(define_insn "*one_cmpl_<optab><mode>2"
[(set (match_operand:GPI 0 "register_operand" "=r")
(not:GPI (SHIFT:GPI (match_operand:GPI 1 "register_operand" "r")

4
gcc/testsuite/ChangeLog
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@ -1,3 +1,7 @@
2020-05-05 Alex Coplan <alex.coplan@arm.com>
* gcc.target/aarch64/mvn_zero_ext.c: New test.
2020-05-05 Jakub Jelinek <jakub@redhat.com>
PR tree-optimization/94800

15
gcc/testsuite/gcc.target/aarch64/mvn_zero_ext.c Normal file
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@ -0,0 +1,15 @@
/* { dg-do compile } */
/* { dg-options "-O2" } */
/*
** foo:
** mvn w0, w0
** ret
*/
unsigned long long
foo (unsigned a)
{
return ~a;
}
/* { dg-final { check-function-bodies "**" "" "" } } */