https://bugs.llvm.org/show_bug.cgi?id=36706
Bug ID: 36706
Summary: [X86] Could commute some VEX encoded instructions to
enable 2 byte VEX prefix
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: craig.top...@gmail.com
CC: llvm-bugs@lists.llvm.org
The VEX prefix comes in two forms, 2 byte and 3 byte. The 2 byte version is
missing some fields from the 3 byte version so can't encode all instructions.
The missing fields are VEX.W, VEX.mmmmm, VEX.X, and VEX.B. VEX.X is only used
by memory instructions. VEX.W isn't used on most instructions.
For a commutable instructions like the register only form of VPADDD, if one
input is xmm8-15 and the other is xmm0-7, we could take advantage of commuting
to always put the xmm8-15 in the operand that uses VEX.R to avoid needing
VEX.B.
This would only work for instructions that use VEX.mmmmm = 0x1, but that's the
more common encoding for that field.
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