https://bugs.llvm.org/show_bug.cgi?id=46094
Bug ID: 46094
Summary: [X86] Improve handling of moving with AH/BH/CH/DH
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: llvm-...@redking.me.uk
CC: andrea.dibia...@gmail.com, craig.top...@gmail.com,
efrie...@quicinc.com, llvm-bugs@lists.llvm.org,
llvm-...@redking.me.uk, spatel+l...@rotateright.com
Based off a discussion https://reviews.llvm.org/D80466
For byte packing cases:
define i16 @pack(i8 %hi, i8 %lo) {
%lox = zext i8 %lo to i16
%hix = zext i8 %hi to i16
%hix_shift = shl i16 %hix, 8
%pack = or i16 %lox, %hix_shift
ret i16 %pack
}
we can either shift+or or use movb to move %hi's byte directly into upper byte
of %lo's register.
movzbl %sil, %eax
shll $8, %edi
orl %edi, %eax
retq
vs
movzbl %sil, %eax
movb %dl, %ah
retq
The pros/cons are incredibly specific to the target hardware, the registers we
have available and what instructions we're feeding the packing operation
outof/into.
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