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Teach jump threading to "look through" a select when the branch direction of a terminator depends on it.
When it sees a promising select it now tries to figure out whether the condition of the select is known in any of the predecessors and if so it maps the operands appropriately. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121859 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Frits van Bommel
parent
22447ae54b
commit
26e097ca4b
@ -538,6 +538,40 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
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}
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}
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if (SelectInst *SI = dyn_cast<SelectInst>(I)) {
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// Handle select instructions where at least one operand is a known constant
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// and we can figure out the condition value for any predecessor block.
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Constant *TrueVal = getKnownConstant(SI->getTrueValue(), Preference);
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Constant *FalseVal = getKnownConstant(SI->getFalseValue(), Preference);
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PredValueInfoTy Conds;
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if ((TrueVal || FalseVal) &&
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ComputeValueKnownInPredecessors(SI->getCondition(), BB, Conds,
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WantInteger)) {
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for (unsigned i = 0, e = Conds.size(); i != e; ++i) {
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Constant *Cond = Conds[i].first;
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// Figure out what value to use for the condition.
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bool KnownCond;
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if (ConstantInt *CI = dyn_cast<ConstantInt>(Cond)) {
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// A known boolean.
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KnownCond = CI->isOne();
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} else {
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assert(isa<UndefValue>(Cond) && "Unexpected condition value");
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// Either operand will do, so be sure to pick the one that's a known
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// constant.
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// FIXME: Do this more cleverly if both values are known constants?
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KnownCond = (TrueVal != 0);
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}
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// See if the select has a known constant value for this predecessor.
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if (Constant *Val = KnownCond ? TrueVal : FalseVal)
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Result.push_back(std::make_pair(Val, Conds[i].second));
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}
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return !Result.empty();
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}
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}
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// If all else fails, see if LVI can figure out a constant value for us.
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Constant *CI = LVI->getConstant(V, BB);
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if (Constant *KC = getKnownConstant(CI, Preference)) {
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123
test/Transforms/JumpThreading/select.ll
Normal file
123
test/Transforms/JumpThreading/select.ll
Normal file
@ -0,0 +1,123 @@
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; RUN: opt -S -jump-threading < %s | FileCheck %s
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declare void @foo()
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declare void @bar()
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declare void @baz()
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declare void @quux()
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; Jump threading of branch with select as condition.
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; Mostly theoretical since instruction combining simplifies all selects of
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; booleans where at least one operand is true/false/undef.
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; CHECK: @test_br
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; CHECK-NEXT: entry:
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; CHECK-NEXT: br i1 %cond, label %L1,
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define void @test_br(i1 %cond, i1 %value) nounwind {
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entry:
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br i1 %cond, label %L0, label %L3
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L0:
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%expr = select i1 %cond, i1 true, i1 %value
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br i1 %expr, label %L1, label %L2
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L1:
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call void @foo()
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ret void
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L2:
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call void @bar()
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ret void
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L3:
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call void @baz()
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br label %L0
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}
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; Jump threading of switch with select as condition.
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; CHECK: @test_switch
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; CHECK-NEXT: entry:
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; CHECK-NEXT: br i1 %cond, label %L1,
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define void @test_switch(i1 %cond, i8 %value) nounwind {
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entry:
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br i1 %cond, label %L0, label %L4
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L0:
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%expr = select i1 %cond, i8 1, i8 %value
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switch i8 %expr, label %L3 [i8 1, label %L1 i8 2, label %L2]
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L1:
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call void @foo()
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ret void
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L2:
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call void @bar()
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ret void
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L3:
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call void @baz()
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ret void
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L4:
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call void @quux()
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br label %L0
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}
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; Make sure the blocks in the indirectbr test aren't trivially removable as
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; successors by taking their addresses.
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@anchor = constant [3 x i8*] [
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i8* blockaddress(@test_indirectbr, %L1),
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i8* blockaddress(@test_indirectbr, %L2),
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i8* blockaddress(@test_indirectbr, %L3)
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]
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; Jump threading of indirectbr with select as address.
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; CHECK: @test_indirectbr
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; CHECK-NEXT: entry:
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; CHECK-NEXT: br i1 %cond, label %L1, label %L3
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define void @test_indirectbr(i1 %cond, i8* %address) nounwind {
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entry:
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br i1 %cond, label %L0, label %L3
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L0:
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%indirect.goto.dest = select i1 %cond, i8* blockaddress(@test_indirectbr, %L1), i8* %address
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indirectbr i8* %indirect.goto.dest, [label %L1, label %L2, label %L3]
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L1:
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call void @foo()
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ret void
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L2:
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call void @bar()
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ret void
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L3:
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call void @baz()
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ret void
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}
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; A more complicated case: the condition is a select based on a comparison.
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; CHECK: @test_switch_cmp
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; CHECK-NEXT: entry:
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; CHECK-NEXT: br i1 %cond, label %L0, label %[[THREADED:[A-Za-z.0-9]+]]
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; CHECK: [[THREADED]]:
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; CHECK-NEXT: call void @quux
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; CHECK-NEXT: br label %L1
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define void @test_switch_cmp(i1 %cond, i32 %val, i8 %value) nounwind {
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entry:
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br i1 %cond, label %L0, label %L4
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L0:
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%val.phi = phi i32 [%val, %entry], [-1, %L4]
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%cmp = icmp slt i32 %val.phi, 0
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%expr = select i1 %cmp, i8 1, i8 %value
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switch i8 %expr, label %L3 [i8 1, label %L1 i8 2, label %L2]
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L1:
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call void @foo()
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ret void
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L2:
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call void @bar()
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ret void
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L3:
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call void @baz()
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ret void
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L4:
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call void @quux()
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br label %L0
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}
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