[PATCH] D58251: Extend "idempotent" atomicrmw optimizations to floating point

[Philip Reames via Phabricator via llvm-commits]

reames created this revision.
reames added reviewers: jfb, qcolombet, spatel.
Herald added subscribers: bollu, mcrosier.

Caution: I am assuming the values used on ConstantExpr::getBinOpIdentity are correct.  If someone with floating point knowledge can confirm, that'd be great.

Extend the integer transform in the obvious way.  I decided not to cast to integer type to be conservative for the moment.  Happy to reverse that if desired.


https://reviews.llvm.org/D58251

Files:
  lib/Transforms/InstCombine/InstCombineAtomicRMW.cpp
  test/Transforms/InstCombine/atomicrmw.ll


Index: test/Transforms/InstCombine/atomicrmw.ll
===================================================================
--- test/Transforms/InstCombine/atomicrmw.ll
+++ test/Transforms/InstCombine/atomicrmw.ll
@@ -67,6 +67,29 @@
   ret i8 %res
 }
 
+; CHECK-LABEL: atomic_fsub
+; CHECK-NEXT: %res = load atomic float, float* %addr monotonic, align 4
+; CHECK-NEXT: ret float %res
+define float @atomic_fsub_zero(float* %addr) {
+  %res = atomicrmw fsub float* %addr, float 0.0 monotonic
+  ret float %res
+}
+
+; CHECK-LABEL: atomic_fadd
+; CHECK-NEXT: %res = load atomic float, float* %addr monotonic, align 4
+; CHECK-NEXT: ret float %res
+define float @atomic_fadd_zero(float* %addr) {
+  %res = atomicrmw fadd float* %addr, float -0.0 monotonic
+  ret float %res
+}
+
+; CHECK-LABEL: atomic_fsub_canon
+; CHECK-NEXT: %res = atomicrmw fadd float* %addr, float -0.000000e+00 release
+; CHECK-NEXT: ret float %res
+define float @atomic_fsub_canon(float* %addr) {
+  %res = atomicrmw fsub float* %addr, float 0.0 release
+  ret float %res
+}
 
 ; Can't replace a volatile w/a load; this would eliminate a volatile store.
 ; CHECK-LABEL: atomic_sub_zero_volatile
Index: lib/Transforms/InstCombine/InstCombineAtomicRMW.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineAtomicRMW.cpp
+++ lib/Transforms/InstCombine/InstCombineAtomicRMW.cpp
@@ -21,12 +21,21 @@
 /// TODO: Common w/ the version in AtomicExpandPass, and change the term used.
 /// Idemptotent is confusing in this context.
 bool isIdempotentRMW(AtomicRMWInst& RMWI) {
+  AtomicRMWInst::BinOp Op = RMWI.getOperation();
+  if (auto CF = dyn_cast<ConstantFP>(RMWI.getValOperand()))
+    switch(Op) {
+    case AtomicRMWInst::FAdd: // -0.0
+      return CF->isZero() && CF->isNegative();
+    case AtomicRMWInst::FSub: // +0.0
+      return CF->isZero() && !CF->isNegative();
+    default:
+      return false;
+    };
+
   auto C = dyn_cast<ConstantInt>(RMWI.getValOperand());
   if(!C)
-    // TODO: Handle fadd, fsub?
     return false;
 
-  AtomicRMWInst::BinOp Op = RMWI.getOperation();
   switch(Op) {
     case AtomicRMWInst::Add:
     case AtomicRMWInst::Sub:
@@ -68,12 +77,18 @@
 
   // We chose to canonicalize all idempotent operations to an single
   // operation code and constant.  This makes it easier for the rest of the
-  // optimizer to match easily.  The choice of or w/zero is arbitrary.
+  // optimizer to match easily.  The choices of or w/0 and fadd w/-0.0 are
+  // arbitrary. 
   if (RMWI.getType()->isIntegerTy() &&
       RMWI.getOperation() != AtomicRMWInst::Or) {
     RMWI.setOperation(AtomicRMWInst::Or);
     RMWI.setOperand(1, ConstantInt::get(RMWI.getType(), 0));
     return &RMWI;
+  } else if (RMWI.getType()->isFloatingPointTy() &&
+             RMWI.getOperation() != AtomicRMWInst::FAdd) {
+    RMWI.setOperation(AtomicRMWInst::FAdd);
+    RMWI.setOperand(1, ConstantFP::getNegativeZero(RMWI.getType()));
+    return &RMWI;
   }
 
   // Check if the required ordering is compatible with an atomic load.


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