[llvm-commits] [llvm] r78812 - in /llvm/trunk: lib/Target/README.txt lib/Transforms/Scalar/InstructionCombining.cpp test/Transforms/InstCombine/nsw.ll

[Dan Gohman]

Author: djg
Date: Wed Aug 12 11:37:02 2009
New Revision: 78812

URL: http://llvm.org/viewvc/llvm-project?rev=78812&view=rev
Log:
Transform -X/C to X/-C, implementing a README.txt entry.

Added:
    llvm/trunk/test/Transforms/InstCombine/nsw.ll
Modified:
    llvm/trunk/lib/Target/README.txt
    llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp

Modified: llvm/trunk/lib/Target/README.txt
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/README.txt?rev=78812&r1=78811&r2=78812&view=diff

==============================================================================
--- llvm/trunk/lib/Target/README.txt (original)
+++ llvm/trunk/lib/Target/README.txt Wed Aug 12 11:37:02 2009
@@ -1111,16 +1111,6 @@
 
 //===---------------------------------------------------------------------===//
 
-We would like to do the following transform in the instcombiner:
-
-  -X/C -> X/-C
-
-However, this isn't valid if (-X) overflows. We can implement this when we
-have the concept of a "C signed subtraction" operator that which is undefined
-on overflow.
-
-//===---------------------------------------------------------------------===//
-
 This was noticed in the entryblock for grokdeclarator in 403.gcc:
 
         %tmp = icmp eq i32 %decl_context, 4          

Modified: llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp?rev=78812&r1=78811&r2=78812&view=diff

==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp Wed Aug 12 11:37:02 2009
@@ -3077,6 +3077,14 @@
                                             RHS->getValue().exactLogBase2());
       return BinaryOperator::CreateAShr(Op0, ShAmt, I.getName());
     }
+
+    // -X/C  -->  X/-C  provided the negation doesn't overflow.
+    if (SubOperator *Sub = dyn_cast<SubOperator>(Op0))
+      if (isa<Constant>(Sub->getOperand(0)) &&
+          cast<Constant>(Sub->getOperand(0))->isNullValue() &&
+          Sub->hasNoSignedOverflow())
+        return BinaryOperator::CreateSDiv(Sub->getOperand(1),
+                                          ConstantExpr::getNeg(RHS));
   }
 
   // If the sign bits of both operands are zero (i.e. we can prove they are

Added: llvm/trunk/test/Transforms/InstCombine/nsw.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/nsw.ll?rev=78812&view=auto

==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/nsw.ll (added)
+++ llvm/trunk/test/Transforms/InstCombine/nsw.ll Wed Aug 12 11:37:02 2009
@@ -0,0 +1,20 @@
+; RUN: llvm-as < %s | opt -instcombine | llvm-dis | FileCheck %s
+
+; CHECK: define i32 @foo
+; %y = sub i32 0, %x
+; %z = sdiv i32 %y, 337
+; ret i32 %y
+define i32 @foo(i32 %x) {
+  %y = sub i32 0, %x
+  %z = sdiv i32 %y, 337
+  ret i32 %y
+}
+
+; CHECK: define i32 @bar
+; %y = sdiv i32 %x, -337
+; ret i32 %y
+define i32 @bar(i32 %x) {
+  %y = sub nsw i32 0, %x
+  %z = sdiv i32 %y, 337
+  ret i32 %y
+}