RFR: 8276673: Optimize abs operations in C2 compiler [v5]
Fei Gao
fgao at openjdk.java.net
Wed Dec 15 15:07:39 UTC 2021
> The patch aims to help optimize Math.abs() mainly from these three parts:
> 1) Remove redundant instructions for abs with constant values
> 2) Remove redundant instructions for abs with char type
> 3) Convert some common abs operations to ideal forms
>
> 1. Remove redundant instructions for abs with constant values
>
> If we can decide the value of the input node for function Math.abs()
> at compile-time, we can substitute the Abs node with the absolute
> value of the constant and don't have to calculate it at runtime.
>
> For example,
> int[] a
> for (int i = 0; i < SIZE; i++) {
> a[i] = Math.abs(-38);
> }
>
> Before the patch, the generated code for the testcase above is:
> ...
> mov w10, #0xffffffda
> cmp w10, wzr
> cneg w17, w10, lt
> dup v16.8h, w17
> ...
> After the patch, the generated code for the testcase above is :
> ...
> movi v16.4s, #0x26
> ...
>
> 2. Remove redundant instructions for abs with char type
>
> In Java semantics, as the char type is always non-negative, we
> could actually remove the absI node in the C2 middle end.
>
> As for vectorization part, in current SLP, the vectorization of
> Math.abs() with char type is intentionally disabled after
> JDK-8261022 because it generates incorrect result before. After
> removing the AbsI node in the middle end, Math.abs(char) can be
> vectorized naturally.
>
> For example,
>
> char[] a;
> char[] b;
> for (int i = 0; i < SIZE; i++) {
> b[i] = (char) Math.abs(a[i]);
> }
>
> Before the patch, the generated assembly code for the testcase
> above is:
>
> B15:
> add x13, x21, w20, sxtw #1
> ldrh w11, [x13, #16]
> cmp w11, wzr
> cneg w10, w11, lt
> strh w10, [x13, #16]
> ldrh w10, [x13, #18]
> cmp w10, wzr
> cneg w10, w10, lt
> strh w10, [x13, #18]
> ...
> add w20, w20, #0x1
> cmp w20, w17
> b.lt B15
>
> After the patch, the generated assembly code is:
> B15:
> sbfiz x18, x19, #1, #32
> add x0, x14, x18
> ldr q16, [x0, #16]
> add x18, x21, x18
> str q16, [x18, #16]
> ldr q16, [x0, #32]
> str q16, [x18, #32]
> ...
> add w19, w19, #0x40
> cmp w19, w17
> b.lt B15
>
> 3. Convert some common abs operations to ideal forms
>
> The patch overrides some virtual support functions for AbsNode
> so that optimization of gvn can work on it. Here are the optimizable
> forms:
>
> a) abs(0 - x) => abs(x)
>
> Before the patch:
> ...
> ldr w13, [x13, #16]
> neg w13, w13
> cmp w13, wzr
> cneg w14, w13, lt
> ...
> After the patch:
> ...
> ldr w13, [x13, #16]
> cmp w13, wzr
> cneg w13, w13, lt
> ...
>
> b) abs(abs(x)) => abs(x)
>
> Before the patch:
> ...
> ldr w12, [x12, #16]
> cmp w12, wzr
> cneg w12, w12, lt
> cmp w12, wzr
> cneg w12, w12, lt
> ...
> After the patch:
> ...
> ldr w13, [x13, #16]
> cmp w13, wzr
> cneg w13, w13, lt
> ...
Fei Gao has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:
- Add a jmh benchmark case
Change-Id: I64938d543126c2e3f9fad8ffc4a50e25e4473d8f
- Merge branch 'master' of github.com:fg1417/jdk into fg8276673
Change-Id: I71987594e9288a489a04de696e69a62f4ad19357
- Merge branch 'master' into fg8276673
Change-Id: I5e3898054b75f49653b8c3b37e4f5007675fa963
- 8276673: Optimize abs operations in C2 compiler
The patch aims to help optimize Math.abs() mainly from these three parts:
1) Remove redundant instructions for abs with constant values
2) Remove redundant instructions for abs with char type
3) Convert some common abs operations to ideal forms
1. Remove redundant instructions for abs with constant values
If we can decide the value of the input node for function Math.abs()
at compile-time, we can substitute the Abs node with the absolute
value of the constant and don't have to calculate it at runtime.
For example,
int[] a
for (int i = 0; i < SIZE; i++) {
a[i] = Math.abs(-38);
}
Before the patch, the generated code for the testcase above is:
...
mov w10, #0xffffffda
cmp w10, wzr
cneg w17, w10, lt
dup v16.8h, w17
...
After the patch, the generated code for the testcase above is :
...
movi v16.4s, #0x26
...
2. Remove redundant instructions for abs with char type
In Java semantics, as the char type is always non-negative, we
could actually remove the absI node in the C2 middle end.
As for vectorization part, in current SLP, the vectorization of
Math.abs() with char type is intentionally disabled after
JDK-8261022 because it generates incorrect result before. After
removing the AbsI node in the middle end, Math.abs(char) can be
vectorized naturally.
For example,
char[] a;
char[] b;
for (int i = 0; i < SIZE; i++) {
b[i] = (char) Math.abs(a[i]);
}
Before the patch, the generated assembly code for the testcase
above is:
B15:
add x13, x21, w20, sxtw #1
ldrh w11, [x13, #16]
cmp w11, wzr
cneg w10, w11, lt
strh w10, [x13, #16]
ldrh w10, [x13, #18]
cmp w10, wzr
cneg w10, w10, lt
strh w10, [x13, #18]
...
add w20, w20, #0x1
cmp w20, w17
b.lt B15
After the patch, the generated assembly code is:
B15:
sbfiz x18, x19, #1, #32
add x0, x14, x18
ldr q16, [x0, #16]
add x18, x21, x18
str q16, [x18, #16]
ldr q16, [x0, #32]
str q16, [x18, #32]
...
add w19, w19, #0x40
cmp w19, w17
b.lt B15
3. Convert some common abs operations to ideal forms
The patch overrides some virtual support functions for AbsNode
so that optimization of gvn can work on it. Here are the optimizable
forms:
a) abs(0 - x) => abs(x)
Before the patch:
...
ldr w13, [x13, #16]
neg w13, w13
cmp w13, wzr
cneg w14, w13, lt
...
After the patch:
...
ldr w13, [x13, #16]
cmp w13, wzr
cneg w13, w13, lt
...
b) abs(abs(x)) => abs(x)
Before the patch:
...
ldr w12, [x12, #16]
cmp w12, wzr
cneg w12, w12, lt
cmp w12, wzr
cneg w12, w12, lt
...
After the patch:
...
ldr w13, [x13, #16]
cmp w13, wzr
cneg w13, w13, lt
...
Change-Id: I5434c01a225796caaf07ffbb19983f4fe2e206bd
-------------
Changes: https://git.openjdk.java.net/jdk/pull/6755/files
Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6755&range=04
Stats: 230 lines in 5 files changed: 223 ins; 2 del; 5 mod
Patch: https://git.openjdk.java.net/jdk/pull/6755.diff
Fetch: git fetch https://git.openjdk.java.net/jdk pull/6755/head:pull/6755
PR: https://git.openjdk.java.net/jdk/pull/6755
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