Remove a very old instcombine where we would turn sequences of selects into

logical operations on the i1's driving them.  This is a bad idea for every
target I can think of (confirmed with micro tests on all of: x86-64, ARM,
AArch64, Mips, and PowerPC) because it forces the i1 to be materialized into
a general purpose register, whereas consuming it directly into a select generally
allows it to exist only transiently in a predicate or flags register.

Chandler ran a set of performance tests with this change, and reported no
measurable change on x86-64.

llvm-svn: 201275

2 files changed, 24 insertions, 25 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 5dd3325a0bc..281ff4050cf 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3313,31 +3313,6 @@ Instruction *InstCombiner::visitFCmpInst(FCmpInst &I) {
         if (Instruction *NV = FoldFCmp_IntToFP_Cst(I, LHSI, RHSC))
           return NV;
         break;
-      case Instruction::Select: {
-        // If either operand of the select is a constant, we can fold the
-        // comparison into the select arms, which will cause one to be
-        // constant folded and the select turned into a bitwise or.
-        Value *Op1 = 0, *Op2 = 0;
-        if (LHSI->hasOneUse()) {
-          if (Constant *C = dyn_cast<Constant>(LHSI->getOperand(1))) {
-            // Fold the known value into the constant operand.
-            Op1 = ConstantExpr::getCompare(I.getPredicate(), C, RHSC);
-            // Insert a new FCmp of the other select operand.
-            Op2 = Builder->CreateFCmp(I.getPredicate(),
-                                      LHSI->getOperand(2), RHSC, I.getName());
-          } else if (Constant *C = dyn_cast<Constant>(LHSI->getOperand(2))) {
-            // Fold the known value into the constant operand.
-            Op2 = ConstantExpr::getCompare(I.getPredicate(), C, RHSC);
-            // Insert a new FCmp of the other select operand.
-            Op1 = Builder->CreateFCmp(I.getPredicate(), LHSI->getOperand(1),
-                                      RHSC, I.getName());
-          }
-        }
-
-        if (Op1)
-          return SelectInst::Create(LHSI->getOperand(0), Op1, Op2);
-        break;
-      }
       case Instruction::FSub: {
         // fcmp pred (fneg x), C -> fcmp swap(pred) x, -C
         Value *Op;
diff --git a/llvm/test/Transforms/InstCombine/select-select.ll b/llvm/test/Transforms/InstCombine/select-select.ll
new file mode 100644
index 00000000000..65820acf07b
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/select-select.ll
@@ -0,0 +1,24 @@
+; RUN: opt -instcombine -S < %s | FileCheck %s
+
+; CHECK: @foo1
+define float @foo1(float %a) #0 {
+; CHECK-NOT: xor
+  %b = fcmp ogt float %a, 0.000000e+00
+  %c = select i1 %b, float %a, float 0.000000e+00
+  %d = fcmp olt float %c, 1.000000e+00
+  %f = select i1 %d, float %c, float 1.000000e+00
+  ret float %f
+}
+
+; CHECK: @foo2
+define float @foo2(float %a) #0 {
+; CHECK-NOT: xor
+  %b = fcmp ogt float %a, 0.000000e+00
+  %c = select i1 %b, float %a, float 0.000000e+00
+  %d = fcmp olt float %c, 1.000000e+00
+  %e = select i1 %b, float %a, float 0.000000e+00
+  %f = select i1 %d, float %e, float 1.000000e+00
+  ret float %f
+}
+
+attributes #0 = { nounwind readnone ssp uwtable }