[InstCombine] convert 'isPositive' and 'isNegative' vector comparisons to shifts (PR26701)

As noted in the code comment, I don't think we can do the same transform that we do for
*scalar* integers comparisons to *vector* integers comparisons because it might pessimize
the general case. 

Exhibit A for an incomplete integer comparison ISA remains x86 SSE/AVX: it only has EQ and GT
for integer vectors.

But we should now recognize all the variants of this construct and produce the optimal code
for the cases shown in:
https://llvm.org/bugs/show_bug.cgi?id=26701
 

llvm-svn: 262424

2 files changed, 45 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 6d0e8821e8f..3cf38c33853 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -976,16 +976,28 @@ Instruction *InstCombiner::transformSExtICmp(ICmpInst *ICI, Instruction &CI) {
     // (x <s  0) ? -1 : 0 -> ashr x, 31        -> all ones if negative
     // (x >s -1) ? -1 : 0 -> not (ashr x, 31)  -> all ones if positive
     if ((Pred == ICmpInst::ICMP_SLT && Op1C->isNullValue()) ||
-        (Pred == ICmpInst::ICMP_SGT && Op1C->isAllOnesValue())) {
+        (Pred == ICmpInst::ICMP_SGT && Op1C->isAllOnesValue()) ||
 
+        // The following patterns should only be present for vectors.
+        // For scalar integers, the comparison should be canonicalized to one of
+        // the above forms. We don't do that for vectors because vector ISAs may
+        // not have a full range of comparison operators. This transform,
+        // however, will simplify the IR, so we always do it.
+        //
+        // (x <=s -1) ? -1 : 0 -> ashr x, 31        -> all ones if negative
+        // (x >=s  0) ? -1 : 0 -> not (ashr x, 31)  -> all ones if positive
+        (Pred == ICmpInst::ICMP_SLE && Op1C->isAllOnesValue()) ||
+        (Pred == ICmpInst::ICMP_SGE && Op1C->isNullValue())) {
       Value *Sh = ConstantInt::get(Op0->getType(),
-                                   Op0->getType()->getScalarSizeInBits()-1);
-      Value *In = Builder->CreateAShr(Op0, Sh, Op0->getName()+".lobit");
+                                   Op0->getType()->getScalarSizeInBits() - 1);
+      Value *In = Builder->CreateAShr(Op0, Sh, Op0->getName() + ".lobit");
       if (In->getType() != CI.getType())
         In = Builder->CreateIntCast(In, CI.getType(), true/*SExt*/);
 
-      if (Pred == ICmpInst::ICMP_SGT)
-        In = Builder->CreateNot(In, In->getName()+".not");
+      // Invert the sign bit if the comparison was checking for 'is positive'.
+      if (Pred == ICmpInst::ICMP_SGT || Pred == ICmpInst::ICMP_SGE)
+        In = Builder->CreateNot(In, In->getName() + ".not");
+
       return replaceInstUsesWith(CI, In);
     }
   }
diff --git a/llvm/test/Transforms/InstCombine/vec_sext.ll b/llvm/test/Transforms/InstCombine/vec_sext.ll
index 6f0d21476e3..670789af050 100644
--- a/llvm/test/Transforms/InstCombine/vec_sext.ll
+++ b/llvm/test/Transforms/InstCombine/vec_sext.ll
@@ -43,3 +43,31 @@ entry:
 ; CHECK:   and <4 x i32> %b.lobit.not, %sub
 ; CHECK:   or <4 x i32> %0, %1
 }
+
+;;; PR26701: https://llvm.org/bugs/show_bug.cgi?id=26701
+
+; Signed-less-than-or-equal to -1 is the same operation as above: smear the sign bit.
+
+define <2 x i32> @is_negative(<2 x i32> %a) {
+  %cmp = icmp sle <2 x i32> %a, <i32 -1, i32 -1>
+  %sext = sext <2 x i1> %cmp to <2 x i32>
+  ret <2 x i32> %sext
+
+; CHECK-LABEL: @is_negative(
+; CHECK-NEXT:  ashr <2 x i32> %a, <i32 31, i32 31>
+; CHECK-NEXT:  ret <2 x i32> 
+}
+
+; Signed-greater-than-or-equal to 0 is 'not' of the same operation as above.
+
+define <2 x i32> @is_positive(<2 x i32> %a) {
+  %cmp = icmp sge <2 x i32> %a, zeroinitializer
+  %sext = sext <2 x i1> %cmp to <2 x i32>
+  ret <2 x i32> %sext
+
+; CHECK-LABEL: @is_positive(
+; CHECK-NEXT:  [[SHIFT:%[a-zA-Z0-9.]+]] = ashr <2 x i32> %a, <i32 31, i32 31>
+; CHECK-NEXT:  xor <2 x i32> [[SHIFT]], <i32 -1, i32 -1>
+; CHECK-NEXT:  ret <2 x i32>
+}
+