X86/TargetTransformInfo: Report div/rem constant immediate costs as T…

…CC_Free

DIV/REM by constants should always be expanded into mul/shift/etc.
patterns. Unfortunately the ConstantHoisting pass runs too early at a
point where the pattern isn't expanded yet. However after
ConstantHoisting hoisted some immediate the result may not expand
anymore. Also the hoisting typically doesn't make sense because it
operates on immediates that will change completely during the expansion.

Report DIV/REM as TCC_Free so ConstantHoisting will not touch them.

Differential Revision: https://reviews.llvm.org/D53174

llvm-svn: 344315

llvm/lib/Target/X86/X86TargetTransformInfo.cpp

@@ -2342,11 +2342,15 @@ int X86TTIImpl::getIntImmCost(unsigned Opcode, unsigned Idx, const APInt &Imm,
       return TTI::TCC_Free;
     ImmIdx = 1;
     break;
-  case Instruction::Mul:
   case Instruction::UDiv:
   case Instruction::SDiv:
   case Instruction::URem:
   case Instruction::SRem:
+    // Division by constant is typically expanded later into a different
+    // instruction sequence. This completely changes the constants.
+    // Report them as "free" to stop ConstantHoist from marking them as opaque.
+    return TTI::TCC_Free;
+  case Instruction::Mul:
   case Instruction::Or:
   case Instruction::Xor:
     ImmIdx = 1;

llvm/test/Transforms/ConstantHoisting/X86/bad-cases.ll

@@ -0,0 +1,47 @@
+; RUN: opt -consthoist -S < %s | FileCheck %s
+target triple = "x86_64--"
+
+; We don't want to convert constant divides because the benefit from converting
+; them to a mul in the backend is larget than constant materialization savings.
+define void @signed_const_division(i64 %in1, i64 %in2, i64* %addr) {
+; CHECK-LABEL: @signed_const_division
+; CHECK: %res1 = sdiv i64 %l1, 4294967296
+; CHECK: %res2 = srem i64 %l2, 4294967296
+entry:
+  br label %loop
+
+loop:
+  %l1 = phi i64 [%res1, %loop], [%in1, %entry]
+  %l2 = phi i64 [%res2, %loop], [%in2, %entry]
+  %res1 = sdiv i64 %l1, 4294967296
+  store volatile i64 %res1, i64* %addr
+  %res2 = srem i64 %l2, 4294967296
+  store volatile i64 %res2, i64* %addr
+  %again = icmp eq i64 %res1, %res2
+  br i1 %again, label %loop, label %end
+
+end:
+  ret void
+}
+
+define void @unsigned_const_division(i64 %in1, i64 %in2, i64* %addr) {
+; CHECK-LABEL: @unsigned_const_division
+; CHECK: %res1 = udiv i64 %l1, 4294967296
+; CHECK: %res2 = urem i64 %l2, 4294967296
+
+entry:
+  br label %loop
+
+loop:
+  %l1 = phi i64 [%res1, %loop], [%in1, %entry]
+  %l2 = phi i64 [%res2, %loop], [%in2, %entry]
+  %res1 = udiv i64 %l1, 4294967296
+  store volatile i64 %res1, i64* %addr
+  %res2 = urem i64 %l2, 4294967296
+  store volatile i64 %res2, i64* %addr
+  %again = icmp eq i64 %res1, %res2
+  br i1 %again, label %loop, label %end
+
+end:
+  ret void
+}