[llvm-dev] [IR canonicalization] 6 ways to choose {-1,0,1}

[Nemanja Ivanovic via llvm-dev]

OK, if it's all the same to everyone else then, I'm in favour of 1/2 from a
code-gen perspective given how we have the PPC back end set up.

On Mon, Jul 10, 2017 at 5:33 PM, Daniel Berlin via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Just to offer some random pieces of data:
> From a GVN/CSE/etc perspective, the non-zext'd versions are the most
> likely to be optimized, and of the non-zext'd versions, the icmp/select is
> the most likely to be optimized.
>
> Especially when combined as part of larger IR.   That is, an "and of
> zext'd icmp and regular icmp" is less likely to be optimized.
> I mean this from the perspective of "be able to deduce later things about
> it".
>
> But past that, i'd say pretty much any of them are fine.
>
>
>
> On Sat, Jul 1, 2017 at 11:45 AM, Sanjay Patel via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> I'm looking at the output of memcmp() expansion (D34904), and I noticed
>> that there are many ways to produce the common positive/zero/negative
>> comparison result in IR.
>>
>> For the following 6 functionally equivalent C source functions, we
>> produce 6 different versions of IR which leads to 6 different asm outputs
>> for x86. Which of these should we choose as canonical IR form?
>>
>> 1. Two selects
>> int zero_negone_one(int x, int y) {
>>   if (x == y) return 0;
>>   if (x < y) return -1;
>>   return 1;
>> }
>>
>> define i32 @zero_negone_one(i32, i32) {
>>   %3 = icmp eq i32 %0, %1
>>   %4 = icmp slt i32 %0, %1
>>   %5 = select i1 %4, i32 -1, i32 1
>>   %6 = select i1 %3, i32 0, i32 %5
>>   ret i32 %6
>> }
>>
>>
>> 2. Two selects, but different
>> int zero_one_negone(int x, int y) {
>>   if (x == y) return 0;
>>   if (x > y) return 1;
>>   return -1;
>> }
>>
>> define i32 @zero_one_negone(i32, i32) {
>>   %3 = icmp eq i32 %0, %1
>>   %4 = icmp sgt i32 %0, %1
>>   %5 = select i1 %4, i32 1, i32 -1
>>   %6 = select i1 %3, i32 0, i32 %5
>>   ret i32 %6
>> }
>>
>>
>> 3. Select and zext
>> int negone_one_zero(int x, int y) {
>>   if (x < y) return -1;
>>   if (x > y) return 1;
>>   return 0;
>> }
>>
>> define i32 @negone_one_zero(i32, i32)  {
>>   %3 = icmp slt i32 %0, %1
>>   %4 = icmp sgt i32 %0, %1
>>   %5 = zext i1 %4 to i32
>>   %6 = select i1 %3, i32 -1, i32 %5
>>   ret i32 %6
>> }
>>
>>
>> 4. Select and sext
>> int negone_zero_one(int x, int y) {
>>   int sel = x < y ? -1 : 0;
>>   if (x > y) return 1;
>>   return sel;
>> }
>>
>> define i32 @negone_zero_one(i32, i32) {
>>   %3 = icmp sgt i32 %0, %1
>>   %4 = icmp slt i32 %0, %1
>>   %5 = sext i1 %4 to i32
>>   %6 = select i1 %3, i32 1, i32 %5
>>   ret i32 %6
>> }
>>
>>
>> 5. Subs and shifts
>> int neg101_sub_shifty(int x, int y) {
>>   int r = (x - y) >> 31;
>>   r += (unsigned)(y - x) >> 31;
>>   return r;
>> }
>>
>> define i32 @neg101_sub_shifty(i32, i32) {
>>   %3 = sub nsw i32 %0, %1
>>   %4 = ashr i32 %3, 31
>>   %5 = sub nsw i32 %1, %0
>>   %6 = lshr i32 %5, 31
>>   %7 = add nsw i32 %4, %6
>>   ret i32 %7
>> }
>>
>>
>> 6. Zexts and sub
>> int neg101_cmp_sub(int x, int y) {
>>   return (x>y) - (x<y);
>> }
>>
>> define i32 @neg101_cmp_sub(i32, i32) {
>>   %3 = icmp sgt i32 %0, %1
>>   %4 = zext i1 %3 to i32
>>   %5 = icmp slt i32 %0, %1
>>   %6 = zext i1 %5 to i32
>>   %7 = sub nsw i32 %4, %6
>>   ret i32 %7
>> }
>>
>>
>> https://godbolt.org/g/UnM9H7
>>
>> Show these are logically equivalent:
>> http://rise4fun.com/Alive/b4D
>>
>> Recent patch related to this pattern:
>> https://reviews.llvm.org/D34278
>>
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>>
>>
>
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