[llvm-commits] [llvm] r126097 - in /llvm/trunk: lib/Support/APInt.cpp lib/Target/X86/README.txt test/CodeGen/X86/divide-by-constant.ll unittests/ADT/APIntTest.cpp

[Cameron Zwarich]

Author: zwarich
Date: Sun Feb 20 18:22:02 2011
New Revision: 126097

URL: http://llvm.org/viewvc/llvm-project?rev=126097&view=rev
Log:
The signed version of our "magic number" computation for the integer approximation
of a constant had a minor typo introduced when copying it from the book, which
caused it to favor negative approximations over positive approximations in many
cases. Positive approximations require fewer operations beyond the multiplication.

In the case of division by 3, we still generate code that is a single instruction
larger than GCC's code.

Modified:
    llvm/trunk/lib/Support/APInt.cpp
    llvm/trunk/lib/Target/X86/README.txt
    llvm/trunk/test/CodeGen/X86/divide-by-constant.ll
    llvm/trunk/unittests/ADT/APIntTest.cpp

Modified: llvm/trunk/lib/Support/APInt.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/APInt.cpp?rev=126097&r1=126096&r2=126097&view=diff
==============================================================================
--- llvm/trunk/lib/Support/APInt.cpp (original)
+++ llvm/trunk/lib/Support/APInt.cpp Sun Feb 20 18:22:02 2011
@@ -1505,7 +1505,7 @@
       r2 = r2 - ad;
     }
     delta = ad - r2;
-  } while (q1.ule(delta) || (q1 == delta && r1 == 0));
+  } while (q1.ult(delta) || (q1 == delta && r1 == 0));
 
   mag.m = q2 + 1;
   if (d.isNegative()) mag.m = -mag.m;   // resulting magic number

Modified: llvm/trunk/lib/Target/X86/README.txt
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/README.txt?rev=126097&r1=126096&r2=126097&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/README.txt (original)
+++ llvm/trunk/lib/Target/X86/README.txt Sun Feb 20 18:22:02 2011
@@ -1888,12 +1888,10 @@
 
 _t:                                     ## @t
 	movslq	%edi, %rax
-	imulq	$-1431655765, %rax, %rcx ## imm = 0xFFFFFFFFAAAAAAAB
-	shrq	$32, %rcx
-	addl	%ecx, %eax
-	movl	%eax, %ecx
-	shrl	$31, %ecx
-	shrl	%eax
+	imulq	$1431655766, %rax, %rax ## imm = 0x55555556
+	movq	%rax, %rcx
+	shrq	$63, %rcx
+	shrq	$32, %rax
 	addl	%ecx, %eax
 	movsbl	%al, %eax
 	ret

Modified: llvm/trunk/test/CodeGen/X86/divide-by-constant.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/divide-by-constant.ll?rev=126097&r1=126096&r2=126097&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/X86/divide-by-constant.ll (original)
+++ llvm/trunk/test/CodeGen/X86/divide-by-constant.ll Sun Feb 20 18:22:02 2011
@@ -40,7 +40,7 @@
 	%div = sdiv i16 %x, 33		; <i32> [#uses=1]
 	ret i16 %div
 ; CHECK: test4:
-; CHECK: imull	$-1985, %ecx, %ecx 
+; CHECK: imull	$1986, %eax, %eax 
 }
 
 define i32 @test5(i32 %A) nounwind {

Modified: llvm/trunk/unittests/ADT/APIntTest.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/ADT/APIntTest.cpp?rev=126097&r1=126096&r2=126097&view=diff
==============================================================================
--- llvm/trunk/unittests/ADT/APIntTest.cpp (original)
+++ llvm/trunk/unittests/ADT/APIntTest.cpp Sun Feb 20 18:22:02 2011
@@ -332,6 +332,24 @@
   EXPECT_EQ(APInt(15, 9).exactLogBase2(), -1);
 }
 
+TEST(APIntTest, magic) {
+  EXPECT_EQ(APInt(32, 3).magic().m, APInt(32, "55555556", 16));
+  EXPECT_EQ(APInt(32, 3).magic().s, 0U);
+  EXPECT_EQ(APInt(32, 5).magic().m, APInt(32, "66666667", 16));
+  EXPECT_EQ(APInt(32, 5).magic().s, 1U);
+  EXPECT_EQ(APInt(32, 7).magic().m, APInt(32, "92492493", 16));
+  EXPECT_EQ(APInt(32, 7).magic().s, 2U);
+}
+
+TEST(APIntTest, magicu) {
+  EXPECT_EQ(APInt(32, 3).magicu().m, APInt(32, "AAAAAAAB", 16));
+  EXPECT_EQ(APInt(32, 3).magicu().s, 1U);
+  EXPECT_EQ(APInt(32, 5).magicu().m, APInt(32, "CCCCCCCD", 16));
+  EXPECT_EQ(APInt(32, 5).magicu().s, 2U);
+  EXPECT_EQ(APInt(32, 7).magicu().m, APInt(32, "24924925", 16));
+  EXPECT_EQ(APInt(32, 7).magicu().s, 3U);
+}
+
 #ifdef GTEST_HAS_DEATH_TEST
 #ifndef NDEBUG
 TEST(APIntTest, StringDeath) {