spirv-opt: Adding folding rules for bitwise and

[alister-chowdhury]

This change adds the following folding ops:

And + Xor/Or

0b1110 & (a | 0b0001) = a & 0b1110
0b1110 & (a ^ 0b0001) = a & 0b1110
0b0110 & (a | 0b1110) = 0b0110

Which should resolve #4902, but since this doesn't add any kind of global variable tracking would be laundered away with any kind of OpPhi.

And + Add/Sub

1 & (b + 2) = b & 1
1 & (b - 2) = b & 1

And + Shifts

1 & (b << 1) = 0
 0x80000000 & (b >> 1) = 0

[s-perron]

Sorry, I only saw the commit with the test changes in the my first review, and that is all I thought it did.

I looked at the changes. One of the rules does not seem correct to me.

Also the tests all uses the same bit patterns, and you are not getting full testing. For the add and subtract cases you need to think about cases with a carry, and what happens if there is overflow or underflow.

[alister-chowdhury]

For the add and subtract cases you need to think about cases with a carry, and what happens if there is overflow or underflow.

My logic was based around an add/subtract can only affect bits >= to its lsb, even with overflow/underflow.

For example (using an 8bit integer as example):

Add

0b11111111 + 0b00000001 = 0b00000000   : bits <= #0 affected
                      ^            ^
0b11111111 + 0b00000010 = 0b00000001   : bits <= #1 affected
                     ^            ^
0b11111111 + 0b00001000 = 0b00000111   : bits <= #3 affected
                   ^            ^

Sub

0b00000000 - 0b00000001 = 0b11111111   : bits >= #0 affected
                      ^            ^
0b00000000 - 0b00000010 = 0b11111110   : bits >= #1 affected
                     ^            ^
0b00000000 - 0b00001000 = 0b11111000   : bits >= #3 affected
                   ^            ^

[s-perron]

Looks good, but a few very minor issues.