[PATCH] D79596: [AMDGPU] Try to determine sign bit during div/rem expansion

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added reviewers: rampitec, arsenm.
Herald added subscribers: llvm-commits, kerbowa, t-tye, tpr, dstuttard, yaxunl, nhaehnle, wdng, jvesely, kzhuravl.
Herald added a project: LLVM.
arsenm accepted this revision.
This revision is now accepted and ready to land.

This is preparation for D79294 <https://reviews.llvm.org/D79294>, which removes an expensive InstSimplify optimization, on the assumption that it will be picked up by InstCombine instead. Of course, this does not hold up if a backend performs non-trivial IR expansions without running a canonicalization pipeline afterwards, which turned up as an issue in the context of AMDGPU div/rem expansion.

This patch mitigates the issue by explicitly performing a known bits calculation where it matters. No test changes, as those would only be visible after the other patch lands.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D79596

Files:
  lib/Target/AMDGPU/AMDGPUCodeGenPrepare.cpp


Index: lib/Target/AMDGPU/AMDGPUCodeGenPrepare.cpp
===================================================================
--- lib/Target/AMDGPU/AMDGPUCodeGenPrepare.cpp
+++ lib/Target/AMDGPU/AMDGPUCodeGenPrepare.cpp
@@ -1005,6 +1005,16 @@
   return false;
 }
 
+static Value *getSign32(Value *V, IRBuilder<> &Builder, const DataLayout *DL) {
+  // Check whether the sign can be determined statically.
+  KnownBits Known = computeKnownBits(V, *DL);
+  if (Known.isNegative())
+    return Constant::getAllOnesValue(V->getType());
+  if (Known.isNonNegative())
+    return Constant::getNullValue(V->getType());
+  return Builder.CreateAShr(V, Builder.getInt32(31));
+}
+
 Value* AMDGPUCodeGenPrepare::expandDivRem32(IRBuilder<> &Builder,
                                             BinaryOperator &I,
                                             Value *Num, Value *Den) const {
@@ -1046,9 +1056,8 @@
 
   Value *Sign = nullptr;
   if (IsSigned) {
-    ConstantInt *K31 = Builder.getInt32(31);
-    Value *LHSign = Builder.CreateAShr(Num, K31);
-    Value *RHSign = Builder.CreateAShr(Den, K31);
+    Value *LHSign = getSign32(Num, Builder, DL);
+    Value *RHSign = getSign32(Den, Builder, DL);
     // Remainder sign is the same as LHS
     Sign = IsDiv ? Builder.CreateXor(LHSign, RHSign) : LHSign;
 


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