[X86] and/or/xor could use 8-bit immediate for more constant arguments

[dzaima]

The functions:

void test_or(long* dst, long x) {
    *dst = x | 200;
}
void test_and(long* dst, long x) {
    *dst = x & -200;
}
void test_xor(long* dst, long x) {
    *dst = x ^ 200;
}

for x86-64 with -O3 (or even -Oz) compile to:

or  rsi, 0xc8               ; 48 81 ce c8 00 00 00
and rsi, 0xffffffffffffff38 ; 48 81 e6 38 ff ff ff
xor rsi, 0xc8               ; 48 81 f6 c8 00 00 00

whereas they could be:

or  sil, 0xc8 ; 40 80 ce c8
and sil, 0x38 ; 40 80 e6 38
xor sil, 0xc8 ; 40 80 f6 c8

This apply:

• for | for an immediate of 128..255;
• for & with an immediate of -256..-129; (x & -256 can furthermore be xor sil,sil)
• for ^ with an immediate of 128..255.

(uops.info says that the (R8l, I8) and (R64, I32) variants in question are largely equivalent)

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[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: dzaima (dzaima)

The functions: ```c void test_or(long* dst, long x) { *dst = x | 200; } void test_and(long* dst, long x) { *dst = x & -200; } void test_xor(long* dst, long x) { *dst = x ^ 200; } ``` for x86-64 with `-O3` (or even `-Oz`) compile to: ```asm or rsi, 0xc8 ; 48 81 ce c8 00 00 00 and rsi, 0xffffffffffffff38 ; 48 81 e6 38 ff ff ff xor rsi, 0xc8 ; 48 81 f6 c8 00 00 00 ``` whereas they could be: ```asm or sil, 0xc8 ; 40 80 ce c8 and sil, 0x38 ; 40 80 e6 38 xor sil, 0xc8 ; 40 80 f6 c8 ``` This apply: - for `|` for an immediate of 128..255; - for `&` with an immediate of -256..-129; (`x & -256` can furthermore be `xor sil,sil`) - for `^` with an immediate of 128..255.

(uops.info says that the (R8l, I8) and (R64, I32) variants in question are largely equivalent)

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