Add peephole optimization to eliminate some mask AND operations.

On KNC, the various vector comparison instructions can optionally
be masked; if a mask is provided, the result is effectively that
the value returned is the AND of the mask with the result of the
comparison.

This change adds an optimization pass to the C++ backend that looks
for vector ANDs where one operand is a comparison and rewrites
them--e.g. "__and(__equal_float(a, b), c)" is changed to
"__equal_float_and_mask(a, b, c)", saving an instruction in the end.

Issue #319.

cbackend.cpp

@@ -13,6 +13,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "ispc.h"
+#include "module.h"
 
 #include <stdio.h>
 
@@ -4365,6 +4366,104 @@ BitcastCleanupPass::runOnBasicBlock(llvm::BasicBlock &bb) {
     return modifiedAny;
 }
 
+///////////////////////////////////////////////////////////////////////////
+// AndCmpCleanupPass
+
+class AndCmpCleanupPass : public llvm::BasicBlockPass {
+public:
+    AndCmpCleanupPass()
+        : BasicBlockPass(ID) { }
+
+    const char *getPassName() const { return "AndCmp Cleanup Pass"; }
+    bool runOnBasicBlock(llvm::BasicBlock &BB);
+
+    static char ID;
+};
+
+char AndCmpCleanupPass::ID = 0;
+
+// Look for ANDs of masks where one of the operands is a vector compare; we
+// can turn these into specialized calls to masked vector compares and
+// thence eliminate the AND.  For example, rather than emitting
+// __and(__less(a, b), c), we will emit __less_and_mask(a, b, c).
+bool
+AndCmpCleanupPass::runOnBasicBlock(llvm::BasicBlock &bb) {
+    bool modifiedAny = false;
+
+ restart:
+    for (llvm::BasicBlock::iterator iter = bb.begin(), e = bb.end(); iter != e; ++iter) {
+        // See if we have an AND instruction
+        llvm::BinaryOperator *bop = llvm::dyn_cast<llvm::BinaryOperator>(&*iter);
+        if (bop == NULL || bop->getOpcode() != llvm::Instruction::And)
+            continue;
+
+        // Make sure it's a vector AND
+        if (llvm::isa<llvm::VectorType>(bop->getType()) == false)
+            continue;
+
+        // We only care about ANDs of the mask type, not, e.g. ANDs of
+        // int32s vectors.
+        if (bop->getType() != LLVMTypes::MaskType)
+            continue;
+
+        // Now see if either of the operands to the AND is a comparison
+        for (int i = 0; i < 2; ++i) {
+            llvm::Value *op = bop->getOperand(i);
+            llvm::CmpInst *opCmp = llvm::dyn_cast<llvm::CmpInst>(op);
+            if (opCmp == NULL)
+                continue;
+
+            // We have a comparison.  However, we also need to make sure
+            // that it's not comparing two mask values; those can't be
+            // simplified to something simpler.
+            if (opCmp->getOperand(0)->getType() == LLVMTypes::MaskType)
+                break;
+
+            // Success!  Go ahead and replace the AND with a call to the
+            // "__and_mask" variant of the comparison function for this
+            // operand.
+            std::string funcName = lPredicateToString(opCmp->getPredicate());
+            funcName += "_";
+            funcName += lTypeToSuffix(opCmp->getOperand(0)->getType());
+            funcName += "_and_mask";
+
+            llvm::Function *andCmpFunc = m->module->getFunction(funcName);
+            if (andCmpFunc == NULL) {
+                // Declare the function if needed; the first two arguments
+                // are the same as the two arguments to the compare we're
+                // replacing and the third argument is the mask type.
+                llvm::Type *cmpOpType = opCmp->getOperand(0)->getType();
+                llvm::Constant *acf = 
+                    m->module->getOrInsertFunction(funcName, LLVMTypes::MaskType,
+                                                   cmpOpType, cmpOpType, 
+                                                   LLVMTypes::MaskType, NULL);
+                andCmpFunc = llvm::dyn_cast<llvm::Function>(acf);
+                Assert(andCmpFunc != NULL);
+                andCmpFunc->setDoesNotThrow(true);
+                andCmpFunc->setDoesNotAccessMemory(true);
+            }
+
+            // Set up the function call to the *_and_mask function; the
+            // mask value passed in is the other operand to the AND.
+            llvm::Value *args[3] = { opCmp->getOperand(0), opCmp->getOperand(1), 
+                                     bop->getOperand(i ^ 1) };
+            llvm::ArrayRef<llvm::Value *> argArray(&args[0], &args[3]);
+            llvm::Instruction *cmpCall = 
+                llvm::CallInst::Create(andCmpFunc, argArray, 
+                                       LLVMGetName(bop, "_and_mask"),
+                                       (llvm::Instruction *)NULL);
+
+            // And replace the original AND instruction with it.
+            llvm::ReplaceInstWithInst(iter, cmpCall);
+
+            modifiedAny = true;
+            goto restart;
+        }
+    }
+
+    return modifiedAny;
+}
+
 ///////////////////////////////////////////////////////////////////////////
 // MaskOpsCleanupPass
 
@@ -4545,6 +4644,7 @@ WriteCXXFile(llvm::Module *module, const char *fn, int vectorWidth,
     pm.add(llvm::createCFGSimplificationPass());   // clean up after lower invoke.
     pm.add(new SmearCleanupPass(module, vectorWidth));
     pm.add(new BitcastCleanupPass());
+    pm.add(new AndCmpCleanupPass());
     pm.add(new MaskOpsCleanupPass(module));
     pm.add(llvm::createDeadCodeEliminationPass()); // clean up after smear pass
 //CO    pm.add(llvm::createPrintModulePass(&fos));