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[DAGCombiner] eliminate setcc bool math when input is low-bit of some value

Description

[DAGCombiner] eliminate setcc bool math when input is low-bit of some value

This patch has the same motivating example as D48466:
define void @foo(i64 %x, i32 %c.0282.in, i32 %d.0280, i32* %ptr0, i32* %ptr1) {

%c.0282 = and i32 %c.0282.in, 268435455
%a16 = lshr i64 32508, %x
%a17 = and i64 %a16, 1
%tobool = icmp eq i64 %a17, 0
%. = select i1 %tobool, i32 1, i32 2
%.286 = select i1 %tobool, i32 27, i32 26
%shr97 = lshr i32 %c.0282, %.
%shl98 = shl i32 %c.0282.in, %.286
%or99 = or i32 %shr97, %shl98
%shr100 = lshr i32 %d.0280, %.
%shl101 = shl i32 %d.0280, %.286
%or102 = or i32 %shr100, %shl101
store i32 %or99, i32* %ptr0
store i32 %or102, i32* %ptr1
ret void

}

...but I'm trying to kill the setcc bool math sooner rather than later.

By matching a larger pattern that includes both the low-bit mask and the trailing add/sub,
we can create a universally good fold because we always eliminate the condition code
intermediate value.

Here are Alive proofs for these (currently instcombine folds the 'add' variants, but
misses the 'sub' patterns):
https://rise4fun.com/Alive/Gsyp

Name: sub of zext cmp mask

%a = and i8 %x, 1
%c = icmp eq i8 %a, 0
%z = zext i1 %c to i32
%r = sub i32 C1, %z
=>
%optional_cast = zext i8 %a to i32
%r = add i32 %optional_cast, C1-1

Name: add of zext cmp mask

%a = and i32 %x, 1
%c = icmp eq i32 %a, 0
%z = zext i1 %c to i8
%r = add i8 %z, C1
=>
%optional_cast = trunc i32 %a to i8
%r = sub i8 C1+1, %optional_cast

All of the tests look like improvements or neutral to me. But it is possible that x86
test+set+bitop is better than what we now show here. I suspect we could do better by
adding another fold for the 'sub' variants.

We start with select-of-constant in IR in the larger motivating test, so that's why I
included tests with selects. Proofs for those variants:
https://rise4fun.com/Alive/Bx1

Name: true const is bigger
Pre: C2 == (C1 + 1)

%a = and i8 %x, 1
%c = icmp eq i8 %a, 0
%r = select i1 %c, i64 C2, i64 C1
=>
%z = zext i8 %a to i64
%r = sub i64 C2, %z

Name: false const is bigger
Pre: C2 == (C1 + 1)

%a = and i8 %x, 1
%c = icmp eq i8 %a, 0
%r = select i1 %c, i64 C1, i64 C2
=>
%z = zext i8 %a to i64
%r = add i64 C1, %z

Differential Revision: https://reviews.llvm.org/D48466

Event Timeline

a.elovikov added inline comments.
/llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
1954

Hi Sanjay,

That generates the following warning:

warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]

It seems the code would behave as expected because string literal is always true, but I think it would be good to add parentheses around "||".

spatel marked an inline comment as done.Jun 25 2018, 5:38 AM
spatel added a subscriber: RKSimon.
spatel added inline comments.
/llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
1954

Thanks (didn't see a warning building with clang).
@RKSimon fixed it with r335451.