128 bit division generates __udivti3 and __umodti3 instead of calling __udivmodti4 once

[danlark1]

Bugzilla Link	47006
Version	trunk
OS	Linux
CC	@topperc,@RKSimon,@nikic,@rotateright

Extended Description

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

[rotateright]

The DivRemPairs pass turns the IR into this:

define i128 @​udiv128(i128 %a, i128 %b) {
%a.frozen = freeze i128 %a
%b.frozen = freeze i128 %b
%quot = udiv i128 %a.frozen, %b.frozen
%1 = mul i128 %quot, %b.frozen
%rem.decomposed = sub i128 %a.frozen, %1
%sum = add i128 %rem.decomposed, %quot
ret i128 %sum
}

That's based on the TTI call:
bool X86TTIImpl::hasDivRemOp(Type *DataType, bool IsSigned)
...returning false for the 128-bit type.

But even if I hack that to return 'true', I see calls:
callq ___divti3
callq ___modti3

Where in optimization do we recognize that the target supports "__udivmodti4" and convert to that call?

[danlark1]

I believe currently we don't recognize __udivmodti4 anywhere, in RuntimeLibcalls.def we don't instrument them at all

HANDLE_LIBCALL(SDIVREM_I8, nullptr)
HANDLE_LIBCALL(SDIVREM_I16, nullptr)
HANDLE_LIBCALL(SDIVREM_I32, nullptr)
HANDLE_LIBCALL(SDIVREM_I64, nullptr)
HANDLE_LIBCALL(SDIVREM_I128, nullptr)
HANDLE_LIBCALL(UDIVREM_I8, nullptr)
HANDLE_LIBCALL(UDIVREM_I16, nullptr)
HANDLE_LIBCALL(UDIVREM_I32, nullptr)
HANDLE_LIBCALL(UDIVREM_I64, nullptr)
HANDLE_LIBCALL(UDIVREM_I128, nullptr)

__udivmodti4 should be presented on every LP64 bit platform, I believe

[danlark1]

Also DivRem is combined in DAGCombiner::useDivRem(SDNode *Node) but

if (!TLI.isTypeLegal(VT) && !TLI.isOperationCustom(DivRemOpc, VT))
return SDValue();

returns false for 128 bit integers