CheckInteger uses an int64_t encoded using a variable width encoding
that is optimized for encoding a number with a lot of leading zeros.
Negative numbers have no leading zeros so use the largest encoding
requiring 9 bytes.
I believe its most like we want to check for positive and negative
numbers near 0. -1 is quite common due to its use in the 'not'
idiom.
To optimize for this, we can borrow an idea from the bitcode format
and move the sign bit to bit 0 with the magnitude stored in the
upper bits. This will drastically increase the number of leading
zeros for small magnitudes. Then we can run this value through
VBR encoding.
This gives a small reduction in the table size on all in tree
targets except VE where size increased by about 300 bytes due
to intrinsic ids now requiring 3 bytes instead of 2. Since the
intrinsic enum space is shared by all targets this an unfortunate
consquence of where VE is currently located in the range.
Reviewed By: RKSimon
Differential Revision: https://reviews.llvm.org/D96317
Tablegen's DAGISelMatcher emits integers in a VBR format,
so if an integer is below 128 it can fit into a single
byte, otherwise high bit is set, next byte is used etc.
MatcherTable is essentially an unsigned char table. When
SelectionDAGISel parses the table it does a reverse translation.
In a situation when numeric value of an integer to emit is
unknown it can be emitted not as OPC_EmitInteger but as
OPC_EmitStringInteger using a symbolic name of the value.
In this situation the value should not exceed 127.
One of the situations when OPC_EmitStringInteger is used is
if we need to emit a subreg into a matcher table. However,
number of subregs can exceed 127. Currently last defined subreg
for AMDGPU is 192. That results in a silent bug in the ISel
with matcher reading from an invalid offset.
Fixed this bug to emit actual VBR encoded value for a subregs
which value exceeds 127.
Differential Revision: https://reviews.llvm.org/D74368
