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[SimplifyCFG] Constant fold a branch implied by it's incoming edge 

The most common use case is when eliminating redundant range checks in an example like the following:
c = a[i+1] + a[i];

Note that all the smarts of the transform (the implication engine) is already in ValueTracking and is tested directly through InstructionSimplify.

Differential Revision: http://reviews.llvm.org/D13040

llvm-svn: 251596
This commit is contained in:
Philip Reames 2015-10-29 03:11:49 +00:00
parent a904e520c2
commit 846e3e41ed
2 changed files with 94 additions and 0 deletions
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13
llvm/lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -2390,6 +2390,19 @@ static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI,
if (CE->canTrap())
return false;
// If BI is reached from the true path of PBI and PBI's condition implies
// BI's condition, we know the direction of the BI branch.
if (PBI->getSuccessor(0) == BI->getParent() &&
isImpliedCondition(PBI->getCondition(), BI->getCondition()) &&
PBI->getSuccessor(0) != PBI->getSuccessor(1) &&
BB->getSinglePredecessor()) {
// Turn this into a branch on constant.
auto *OldCond = BI->getCondition();
BI->setCondition(ConstantInt::getTrue(BB->getContext()));
RecursivelyDeleteTriviallyDeadInstructions(OldCond);
return true; // Nuke the branch on constant.
}
// If this is a conditional branch in an empty block, and if any
// predecessors are a conditional branch to one of our destinations,
// fold the conditions into logical ops and one cond br.

81
llvm/test/Transforms/SimplifyCFG/implied-cond.ll Normal file
View File

@ -0,0 +1,81 @@
; RUN: opt %s -S -simplifycfg | FileCheck %s
; Check for when one branch implies the value of a successors conditional and
; it's not simply the same conditional repeated.
define void @test(i32 %length.i, i32 %i) {
; CHECK-LABEL: @test
%iplus1 = add nsw i32 %i, 1
%var29 = icmp slt i32 %iplus1, %length.i
; CHECK: br i1 %var29, label %in_bounds, label %out_of_bounds
br i1 %var29, label %next, label %out_of_bounds
next:
; CHECK-LABEL: in_bounds:
; CHECK-NEXT: ret void
%var30 = icmp slt i32 %i, %length.i
br i1 %var30, label %in_bounds, label %out_of_bounds2
in_bounds:
ret void
out_of_bounds:
call void @foo(i64 0)
unreachable
out_of_bounds2:
call void @foo(i64 1)
unreachable
}
; If the add is not nsw, it's not safe to use the fact about i+1 to imply the
; i condition since it could have overflowed.
define void @test_neg(i32 %length.i, i32 %i) {
; CHECK-LABEL: @test_neg
%iplus1 = add i32 %i, 1
%var29 = icmp slt i32 %iplus1, %length.i
; CHECK: br i1 %var29, label %next, label %out_of_bounds
br i1 %var29, label %next, label %out_of_bounds
next:
%var30 = icmp slt i32 %i, %length.i
; CHECK: br i1 %var30, label %in_bounds, label %out_of_bounds2
br i1 %var30, label %in_bounds, label %out_of_bounds2
in_bounds:
ret void
out_of_bounds:
call void @foo(i64 0)
unreachable
out_of_bounds2:
call void @foo(i64 1)
unreachable
}
define void @test2(i32 %length.i, i32 %i) {
; CHECK-LABEL: @test2
%iplus100 = add nsw i32 %i, 100
%var29 = icmp slt i32 %iplus100, %length.i
; CHECK: br i1 %var29, label %in_bounds, label %out_of_bounds
br i1 %var29, label %next, label %out_of_bounds
next:
%var30 = icmp slt i32 %i, %length.i
br i1 %var30, label %in_bounds, label %out_of_bounds2
in_bounds:
ret void
out_of_bounds:
call void @foo(i64 0)
unreachable
out_of_bounds2:
call void @foo(i64 1)
unreachable
}
declare void @foo(i64)