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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.

llvm-svn: 126097
This commit is contained in:
Cameron Zwarich 2011-02-21 00:22:02 +00:00
parent 468b5cbd40
commit 8731d0cc83
4 changed files with 24 additions and 8 deletions
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2
llvm/lib/Support/APInt.cpp
View File

@ -1505,7 +1505,7 @@ APInt::ms APInt::magic() const {
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

10
llvm/lib/Target/X86/README.txt
View File

@ -1888,12 +1888,10 @@ Compiles to: $clang t.c -S -o - -O3 -mkernel -fomit-frame-pointer
_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

2
llvm/test/CodeGen/X86/divide-by-constant.ll
View File

@ -40,7 +40,7 @@ entry:
%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 {

18
llvm/unittests/ADT/APIntTest.cpp
View File

@ -332,6 +332,24 @@ TEST(APIntTest, Log2) {
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) {