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<div class="moz-cite-prefix">The code change LGTM.<br>
<br>
The changes inspires two thoughts:<br>
1) As we've discussed off list and Chandler mentioned, we need to
investigate profitability of various invariant condition
propagation schemes. <br>
2) When do we remove a llvm.assume? In your test case, you had an
assume which (after the transform) retained a condition about an
otherwise used value (%a). This seems like an obvious case to
remove since we've gotten all the benefit we can from it. Are
there other cases like this we should implement?<br>
<br>
p.s. Do you have a wiki or notes page somewhere where we can track
potential enhancements? I worry large parts are getting lost in
all the various review threads.<br>
<br>
Philip<br>
<br>
On 07/17/2014 10:58 AM, <a class="moz-txt-link-abbreviated" href="mailto:hfinkel@anl.gov">hfinkel@anl.gov</a> wrote:<br>
</div>
<blockquote
cite="mid:differential-rev-PHID-DREV-x274oe4o3niz2vh5v5y7-req@reviews.llvm.org"
type="cite">
<pre wrap="">Hi chandlerc,
>From a combination of invariants (and perhaps through other means), it is possible that all bits of a return value might be known. Currently, InstCombine does not check for this (which is understandable given assumptions of constant propagation), but means that we'd miss simple cases where invariants are involved.
<a class="moz-txt-link-freetext" href="http://reviews.llvm.org/D4567">http://reviews.llvm.org/D4567</a>
Files:
lib/Transforms/InstCombine/InstCombine.h
lib/Transforms/InstCombine/InstructionCombining.cpp
test/Transforms/InstCombine/invariants.ll
Index: lib/Transforms/InstCombine/InstCombine.h
===================================================================
--- lib/Transforms/InstCombine/InstCombine.h
+++ lib/Transforms/InstCombine/InstCombine.h
@@ -217,6 +217,7 @@
Instruction *visitStoreInst(StoreInst &SI);
Instruction *visitBranchInst(BranchInst &BI);
Instruction *visitSwitchInst(SwitchInst &SI);
+ Instruction *visitReturnInst(ReturnInst &RI);
Instruction *visitInsertValueInst(InsertValueInst &IV);
Instruction *visitInsertElementInst(InsertElementInst &IE);
Instruction *visitExtractElementInst(ExtractElementInst &EI);
Index: lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- lib/Transforms/InstCombine/InstructionCombining.cpp
+++ lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1923,7 +1923,25 @@
return nullptr;
}
+Instruction *InstCombiner::visitReturnInst(ReturnInst &RI) {
+ if (RI.getNumOperands() == 0) // ret void
+ return nullptr;
+
+ Value *ResultOp = RI.getOperand(0);
+ Type *VTy = ResultOp->getType();
+ if (!VTy->isIntegerTy())
+ return nullptr;
+ // There might be invariants dominating this return that completely determine
+ // the value. If so, constant fold it.
+ unsigned BitWidth = VTy->getPrimitiveSizeInBits();
+ APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
+ computeKnownBits(ResultOp, KnownZero, KnownOne, 0, &RI);
+ if ((KnownZero|KnownOne).isAllOnesValue())
+ RI.setOperand(0, Constant::getIntegerValue(VTy, KnownOne));
+
+ return nullptr;
+}
Instruction *InstCombiner::visitBranchInst(BranchInst &BI) {
// Change br (not X), label True, label False to: br X, label False, True
Index: test/Transforms/InstCombine/invariants.ll
===================================================================
--- test/Transforms/InstCombine/invariants.ll
+++ test/Transforms/InstCombine/invariants.ll
@@ -43,6 +43,18 @@
; Function Attrs: nounwind
declare void @llvm.invariant(i1) #1
+define i32 @simple(i32 %a) #1 {
+entry:
+
+; CHECK-LABEL: @simple
+; CHECK: call void @llvm.invariant
+; CHECK: ret i32 4
+
+ %cmp = icmp eq i32 %a, 4
+ tail call void @llvm.invariant(i1 %cmp)
+ ret i32 %a
+}
+
; Function Attrs: nounwind uwtable
define i32 @bar1(i32 %a) #0 {
entry:
</pre>
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</pre>
</blockquote>
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