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llvm-mirror/test/Transforms/JumpThreading/induction.ll
Philip Reames 965addef14 [LVI] Introduce an intersect operation on lattice values
LVI has several separate sources of facts - edge local conditions, recursive queries, assumes, and control independent value facts - which all apply to the same value at the same location. The existing implementation was very conservative about exploiting all of these facts at once.

This change introduces an "intersect" function specifically to abstract the action of picking a good set of facts from all of the separate facts given. At the moment, this function is relatively simple (i.e. mostly just reuses the bits which were already there), but even the minor additions reveal the inherent power. For example, JumpThreading is now capable of doing an inductive proof that a particular value is always positive and removing a half range check.

I'm currently only using the new intersect function in one place. If folks are happy with the direction of the work, I plan on making a series of small changes without review to replace mergeIn with intersect at all the appropriate places.

Differential Revision: http://reviews.llvm.org/D14476

llvm-svn: 259461
2016-02-02 03:15:40 +00:00

26 lines
596 B
LLVM

; RUN: opt -S -jump-threading < %s | FileCheck %s
define i8 @test(i32 %a, i32 %length) {
; CHECK-LABEL: @test
entry:
; CHECK: br label %backedge
br label %loop
loop:
; CHECK-LABEL: backedge:
; CHECK: phi i32
; CHECK: br i1 %cont, label %backedge, label %exit
%iv = phi i32 [0, %entry], [%iv.next, %backedge]
;; We can use an inductive argument to prove %iv is always positive
%cnd = icmp sge i32 %iv, 0
br i1 %cnd, label %backedge, label %exit
backedge:
%iv.next = add nsw i32 %iv, 1
%cont = icmp slt i32 %iv.next, 400
br i1 %cont, label %loop, label %exit
exit:
ret i8 0
}