https://bugs.llvm.org/show_bug.cgi?id=47006
Bug ID: 47006
Summary: 128 bit division generates __udivti3 and __umodti3
instead of calling __udivmodti4 once
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: kutdan...@yandex.ru
CC: craig.top...@gmail.com, llvm-bugs@lists.llvm.org,
llvm-...@redking.me.uk, spatel+l...@rotateright.com
128 bit division generates __udivti3 and __umodti3 instead of calling
__udivmodti4 once
This happens because of DivRemPairs pass and lack of instrumentation in the
backend.
; Unsigned 128-bit division
define i128 @udiv128(i128 %a, i128 %b) {
%quot = udiv i128 %a, %b
%rem = urem i128 %a, %b
%sum = add i128 %quot, %rem
ret i128 %sum
}
=>
https://gcc.godbolt.org/z/PorhMz
Will call __udivti3 on LP64 but libgcc and compiler-rt have __udivmodti4 which
computes the quotient and the remainder at the same time. This particular hurts
x86 as divq instruction is presented. Other backends can also benefit from this
too
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