https://bugs.llvm.org/show_bug.cgi?id=52267
Bug ID: 52267
Summary: [x86] convert xor with SMIN to add
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: spatel+l...@rotateright.com
CC: craig.top...@gmail.com, llvm-bugs@lists.llvm.org,
llvm-...@redking.me.uk, pengfei.w...@intel.com,
spatel+l...@rotateright.com
Based on discussion in https://reviews.llvm.org/D112085 :
define i32 @add_smin(i32 %x) {
%r = add i32 %x, 2147483648
ret i32 %r
}
define i32 @xor_smin(i32 %x) {
%r = xor i32 %x, 2147483648
ret i32 %r
}
These are logically equivalent:
https://alive2.llvm.org/ce/z/qV46E2
...and instcombine chooses 'xor' for better analysis in IR. But x86 (probably
unlike most targets), may benefit from converting it back to 'add' (for legal
integer types) because that can be lowered as LEA:
movl %edi, %eax
xorl $-2147483648, %eax ## imm = 0x80000000
vs.
leal -2147483648(%rdi), %eax
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