forked from OSchip/llvm-project
7ac2db6a48
We can always eliminate the shift in: icmp gt/lt (shr X, C1), C2 --> icmp gt/lt X, C'
This patch was supposed to just be an efficiency improvement because we were doing this 3-step process to fold:
IC: Visiting: %c = icmp ugt i4 %s, 1
IC: ADD: %s = lshr i4 %x, 1
IC: ADD: %1 = udiv i4 %x, 2
IC: Old = %c = icmp ugt i4 %1, 1
New = <badref> = icmp uge i4 %x, 4
IC: ADD: %c = icmp uge i4 %x, 4
IC: ERASE %2 = icmp ugt i4 %1, 1
IC: Visiting: %c = icmp uge i4 %x, 4
IC: Old = %c = icmp uge i4 %x, 4
New = <badref> = icmp ugt i4 %x, 3
IC: ADD: %c = icmp ugt i4 %x, 3
IC: ERASE %2 = icmp uge i4 %x, 4
IC: Visiting: %c = icmp ugt i4 %x, 3
IC: DCE: %1 = udiv i4 %x, 2
IC: ERASE %1 = udiv i4 %x, 2
IC: DCE: %s = lshr i4 %x, 1
IC: ERASE %s = lshr i4 %x, 1
IC: Visiting: ret i1 %c
When we could go directly to canonical icmp form:
IC: Visiting: %c = icmp ugt i4 %s, 1
IC: Old = %c = icmp ugt i4 %s, 1
New = <badref> = icmp ugt i4 %x, 3
IC: ADD: %c = icmp ugt i4 %x, 3
IC: ERASE %1 = icmp ugt i4 %s, 1
IC: ADD: %s = lshr i4 %x, 1
IC: DCE: %s = lshr i4 %x, 1
IC: ERASE %s = lshr i4 %x, 1
IC: Visiting: %c = icmp ugt i4 %x, 3
...but then I noticed that the folds were incomplete too:
https://godbolt.org/g/aB2hLE
Here are attempts to prove the logic with Alive:
https://rise4fun.com/Alive/92o
Name: lshr_ult
Pre: ((C2 << C1) u>> C1) == C2
%sh = lshr i8 %x, C1
%r = icmp ult i8 %sh, C2
=>
%r = icmp ult i8 %x, (C2 << C1)
Name: ashr_slt
Pre: ((C2 << C1) >> C1) == C2
%sh = ashr i8 %x, C1
%r = icmp slt i8 %sh, C2
=>
%r = icmp slt i8 %x, (C2 << C1)
Name: lshr_ugt
Pre: (((C2+1) << C1) u>> C1) == (C2+1)
%sh = lshr i8 %x, C1
%r = icmp ugt i8 %sh, C2
=>
%r = icmp ugt i8 %x, ((C2+1) << C1) - 1
Name: ashr_sgt
Pre: (C2 != 127) && ((C2+1) << C1 != -128) && (((C2+1) << C1) >> C1) == (C2+1)
%sh = ashr i8 %x, C1
%r = icmp sgt i8 %sh, C2
=>
%r = icmp sgt i8 %x, ((C2+1) << C1) - 1
Name: ashr_exact_sgt
Pre: ((C2 << C1) >> C1) == C2
%sh = ashr exact i8 %x, C1
%r = icmp sgt i8 %sh, C2
=>
%r = icmp sgt i8 %x, (C2 << C1)
Name: ashr_exact_slt
Pre: ((C2 << C1) >> C1) == C2
%sh = ashr exact i8 %x, C1
%r = icmp slt i8 %sh, C2
=>
%r = icmp slt i8 %x, (C2 << C1)
Name: lshr_exact_ugt
Pre: ((C2 << C1) u>> C1) == C2
%sh = lshr exact i8 %x, C1
%r = icmp ugt i8 %sh, C2
=>
%r = icmp ugt i8 %x, (C2 << C1)
Name: lshr_exact_ult
Pre: ((C2 << C1) u>> C1) == C2
%sh = lshr exact i8 %x, C1
%r = icmp ult i8 %sh, C2
=>
%r = icmp ult i8 %x, (C2 << C1)
We did something similar for 'shl' in D28406.
Differential Revision: https://reviews.llvm.org/D38514
llvm-svn: 315021
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