Transform: "icmp eq (trunc (lshr(X, cst1)), cst" to "icmp (and X, mask), cst"

when X has multiple uses.  This is useful for exposing secondary optimizations,
but the X86 backend isn't ready for this when X has a single use.  For example,
this can disable load folding.

This is inching towards resolving PR6627.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130238 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/README.txt

@@ -2259,52 +2259,6 @@ icmp transform.
 
 //===---------------------------------------------------------------------===//
 
-We should optimize this:
-
-  %tmp = load i16* %arrayidx, align 4, !tbaa !0
-  %A = trunc i16 %tmp to i8
-  %cmp = icmp eq i8 %A, 127
-  %B.mask = and i16 %tmp, -256
-  %cmp7 = icmp eq i16 %B.mask, 17664
-  %or.cond = and i1 %cmp, %cmp7
-  br i1 %or.cond, label %land.lhs.true9, label %if.end
-
-into:
-
-  %tmp = load i16* %arrayidx, align 4, !tbaa !0
-  %0 = icmp eq i16 %tmp, 17791
-  br i1 %0, label %land.lhs.true9, label %if.end
-
-with this patch:
-Index: InstCombine/InstCombineCompares.cpp
-===================================================================
---- InstCombine/InstCombineCompares.cpp	(revision 129500)
-+++ InstCombine/InstCombineCompares.cpp	(working copy)
-@@ -2506,6 +2506,18 @@
-         return &I;
-       }
-     }
-+    
-+    // Transform "icmp eq (trunc X), cst" to "icmp (and X, mask), cst"
-+    if (Op0->hasOneUse() && match(Op0, m_Trunc(m_Value(A))) &&
-+        isa<ConstantInt>(Op1)) {
-+      APInt MaskV = APInt::getLowBitsSet(A->getType()->getPrimitiveSizeInBits(),
-+                                      Op0->getType()->getPrimitiveSizeInBits());
-+      Value *Mask =
-+        Builder->CreateAnd(A, ConstantInt::get(A->getContext(), MaskV));
-+      return new ICmpInst(I.getPredicate(), Mask,
-+                          ConstantExpr::getZExt(cast<ConstantInt>(Op1),
-+                                                Mask->getType()));
-+    }
-   }
-   
-   {
-
-
-Not having this is blocking resolving PR6627.
-
-//===---------------------------------------------------------------------===//
-
 This code:
 
 typedef struct {

lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -2506,6 +2506,31 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
       }
     }
     
+    // Transform "icmp eq (trunc (lshr(X, cst1)), cst" to
+    // "icmp (and X, mask), cst"
+    uint64_t ShAmt = 0;
+    ConstantInt *Cst1;
+    if (Op0->hasOneUse() &&
+        match(Op0, m_Trunc(m_OneUse(m_LShr(m_Value(A),
+                                           m_ConstantInt(ShAmt))))) &&
+        match(Op1, m_ConstantInt(Cst1)) &&
+        // Only do this when A has multiple uses.  This is most important to do
+        // when it exposes other optimizations.
+        !A->hasOneUse()) {
+      unsigned ASize =cast<IntegerType>(A->getType())->getPrimitiveSizeInBits();
+      
+      if (ShAmt < ASize) {
+        APInt MaskV =
+          APInt::getLowBitsSet(ASize, Op0->getType()->getPrimitiveSizeInBits());
+        MaskV <<= ShAmt;
+        
+        APInt CmpV = Cst1->getValue().zext(ASize);
+        CmpV <<= ShAmt;
+        
+        Value *Mask = Builder->CreateAnd(A, Builder->getInt(MaskV));
+        return new ICmpInst(I.getPredicate(), Mask, Builder->getInt(CmpV));
+      }
+    }
   }
   
   {

test/Transforms/InstCombine/icmp.ll

@@ -494,3 +494,19 @@ define i1 @test51(i32 %X, i32 %Y) {
   %C = icmp sgt i32 %B, -1
   ret i1 %C
 }
+
+; CHECK: @test52
+; CHECK-NEXT: and i32 %x1, 16711935
+; CHECK-NEXT: icmp eq i32 {{.*}}, 4980863
+; CHECK-NEXT: ret i1
+define i1 @test52(i32 %x1) nounwind {
+  %conv = and i32 %x1, 255
+  %cmp = icmp eq i32 %conv, 127
+  %tmp2 = lshr i32 %x1, 16
+  %tmp3 = trunc i32 %tmp2 to i8
+  %cmp15 = icmp eq i8 %tmp3, 76
+
+  %A = and i1 %cmp, %cmp15
+  ret i1 %A
+}
+