[PATCH] D34141: [X86] Recognize constant arrays with special values and replace loads from it with subtract and shift instructions, which then will be replaced by X86 BZHI machine instruction.

[Ayman Musa via Phabricator via llvm-commits]

aymanmus marked an inline comment as done.
aymanmus added inline comments.


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Comment at: lib/Target/X86/X86ISelLowering.cpp:32521
+    if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(MemOp)) {
+      if (GlobalVariable *GV = dyn_cast<GlobalVariable>(GEP->getOperand(0))) {
+        if (GV->isConstant() && GV->hasDefinitiveInitializer()) {
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m_zuckerman wrote:
> Can you explain why the element must to be a global variable? 
Clang will always hoist a local constant array to be global in LLVM IR.


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Comment at: lib/Target/X86/X86ISelLowering.cpp:32526
+          Type *Ty = Init->getType();
+          if (!isa<ConstantDataArray>(Init) ||
+              !Ty->getArrayElementType()->isIntegerTy() ||
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m_zuckerman wrote:
>   # please clarify this if (add comments) 
>   # Why you need also the isa<ConstantDataArray>(Init) ,is it not the same as GV->isConstant()? 
The second condition makes sure that it is a constant data *array*. GV->isConstant does not.


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Comment at: lib/Target/X86/X86ISelLowering.cpp:32548
+
+          // Do the transformation (For 32-bit type):
+          // -> (and (load arr[idx]), inp)
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m_zuckerman wrote:
> What about 64 bit? 
It's only an example for the 32-bit case, the same happens for 64 as well.


https://reviews.llvm.org/D34141