[PATCH] D26088: Don't leave unused divs/rems sitting around in BypassSlowDivision.

[Justin Lebar via llvm-commits]

jlebar updated this revision to Diff 76243.
jlebar marked an inline comment as done.
jlebar added a comment.
Herald added a subscriber: jholewinski.

Use helper function to delete dead code, and add an IR test.


https://reviews.llvm.org/D26088

Files:
  llvm/lib/Transforms/Utils/BypassSlowDivision.cpp
  llvm/test/Transforms/CodeGenPrepare/NVPTX/bypass-slow-div.ll


Index: llvm/test/Transforms/CodeGenPrepare/NVPTX/bypass-slow-div.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/CodeGenPrepare/NVPTX/bypass-slow-div.ll
@@ -0,0 +1,29 @@
+; RUN: opt -S -codegenprepare < %s | FileCheck %s
+
+target datalayout = "e-i64:64-v16:16-v32:32-n16:32:64"
+target triple = "nvptx64-nvidia-cuda"
+
+; We only use the div instruction -- the rem should be DCE'ed.
+; CHECK-LABEL: @div_only
+define void @div_only(i64 %a, i64 %b, i64* %retptr) {
+  ; CHECK: udiv i32
+  ; CHECK-NOT: urem
+  ; CHECK: sdiv i64
+  ; CHECK-NOT: rem
+  %d = sdiv i64 %a, %b
+  store i64 %d, i64* %retptr
+  ret void
+}
+
+; We only use the rem instruction -- the div should be DCE'ed.
+; CHECK-LABEL: @rem_only
+define void @rem_only(i64 %a, i64 %b, i64* %retptr) {
+  ; CHECK-NOT: div
+  ; CHECK: urem i32
+  ; CHECK-NOT: div
+  ; CHECK: rem i64
+  ; CHECK-NOT: div
+  %d = srem i64 %a, %b
+  store i64 %d, i64* %retptr
+  ret void
+}
Index: llvm/lib/Transforms/Utils/BypassSlowDivision.cpp
===================================================================
--- llvm/lib/Transforms/Utils/BypassSlowDivision.cpp
+++ llvm/lib/Transforms/Utils/BypassSlowDivision.cpp
@@ -20,6 +20,7 @@
 #include "llvm/IR/Function.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/Instructions.h"
+#include "llvm/Transforms/Utils/Local.h"
 
 using namespace llvm;
 
@@ -246,5 +247,12 @@
     MadeChange |= reuseOrInsertFastDiv(I, BT, UseDivOp, UseSignedOp, DivCache);
   }
 
+  // Above we eagerly create divs and rems, as pairs, so that we can efficiently
+  // create divrem machine instructions.  Now erase any unused divs / rems so we
+  // don't leave extra instructions sitting around.
+  for (auto &KV : DivCache)
+    for (Instruction *Phi : {KV.second.Quotient, KV.second.Remainder})
+      RecursivelyDeleteTriviallyDeadInstructions(Phi);
+
   return MadeChange;
 }


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