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Peer through zext and sext to eliminate them when it is safe to do so.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82389 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Nick Lewycky 2009-09-20 07:31:25 +00:00
parent b3c8547cb8
commit 149cbc2a24
3 changed files with 19 additions and 22 deletions
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22
lib/Target/README.txt
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@ -1656,25 +1656,3 @@ Instcombine should be able to optimize away the loads (and thus the globals).
See also PR4973
//===---------------------------------------------------------------------===//
I saw this constant expression in real code after llvm-g++ -O2:
declare extern_weak i32 @0(i64)
define void @foo() {
br i1 icmp eq (i32 zext (i1 icmp ne (i32 (i64)* @0, i32 (i64)* null) to i32),
i32 0), label %cond_true, label %cond_false
cond_true:
ret void
cond_false:
ret void
}
That branch expression should be reduced to:
i1 icmp eq (i32 (i64)* @0, i32 (i64)* null)
It's probably not a perf issue, I just happened to see it while examining
something else and didn't want to forget about it.
//===---------------------------------------------------------------------===//

16
lib/VMCore/ConstantFold.cpp
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@ -1665,6 +1665,22 @@ Constant *llvm::ConstantFoldCompareInstruction(LLVMContext &Context,
}
}
// If the left hand side is an extension, try eliminating it.
if (ConstantExpr *CE1 = dyn_cast<ConstantExpr>(C1)) {
if (CE1->getOpcode() == Instruction::SExt ||
CE1->getOpcode() == Instruction::ZExt) {
Constant *CE1Op0 = CE1->getOperand(0);
Constant *CE1Inverse = ConstantExpr::getTrunc(CE1, CE1Op0->getType());
if (CE1Inverse == CE1Op0) {
// Check whether we can safely truncate the right hand side.
Constant *C2Inverse = ConstantExpr::getTrunc(C2, CE1Op0->getType());
if (ConstantExpr::getZExt(C2Inverse, C2->getType()) == C2) {
return ConstantExpr::getICmp(pred, CE1Inverse, C2Inverse);
}
}
}
}
if (!isa<ConstantExpr>(C1) && isa<ConstantExpr>(C2)) {
// If C2 is a constant expr and C1 isn't, flip them around and fold the
// other way if possible.

3
test/Transforms/ConstProp/2009-09-19-ConstFold-i1-ConstExpr.ll
View File

@ -36,3 +36,6 @@
; CHECK: @M = global i1 icmp uge (i8* @X, i8* @Y) ; <i1*> [#uses=0]
@N = global i1 icmp ne (i1 icmp ult (i8* @X, i8* @Y), i1 false)
; CHECK: @N = global i1 icmp ult (i8* @X, i8* @Y) ; <i1*> [#uses=0]
@O = global i1 icmp eq (i32 zext (i1 icmp ult (i8* @X, i8* @Y) to i32), i32 0)
; CHECK: @O = global i1 icmp uge (i8* @X, i8* @Y) ; <i1*> [#uses=0]