[llvm] r344315 - X86/TargetTransformInfo: Report div/rem constant immediate costs as TCC_Free

[Matthias Braun via llvm-commits]

Author: matze
Date: Thu Oct 11 16:14:35 2018
New Revision: 344315

URL: http://llvm.org/viewvc/llvm-project?rev=344315&view=rev
Log:
X86/TargetTransformInfo: Report div/rem constant immediate costs as TCC_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

Added:
    llvm/trunk/test/Transforms/ConstantHoisting/X86/bad-cases.ll
Modified:
    llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp

Modified: llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp?rev=344315&r1=344314&r2=344315&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp (original)
+++ llvm/trunk/lib/Target/X86/X86TargetTransformInfo.cpp Thu Oct 11 16:14:35 2018
@@ -2342,11 +2342,15 @@ int X86TTIImpl::getIntImmCost(unsigned O
       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;

Added: llvm/trunk/test/Transforms/ConstantHoisting/X86/bad-cases.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/ConstantHoisting/X86/bad-cases.ll?rev=344315&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/ConstantHoisting/X86/bad-cases.ll (added)
+++ llvm/trunk/test/Transforms/ConstantHoisting/X86/bad-cases.ll Thu Oct 11 16:14:35 2018
@@ -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
+}