LowerBitSets: Use byte arrays instead of bit sets to represent in-memory bit sets.

By loading from indexed offsets into a byte array and applying a mask, a
program can test bits from the bit set with a relatively short instruction
sequence. For example, suppose we have 15 bit sets to lay out:

A (16 bits), B (15 bits), C (14 bits), D (13 bits), E (12 bits),
F (11 bits), G (10 bits), H (9 bits), I (7 bits), J (6 bits), K (5 bits),
L (4 bits), M (3 bits), N (2 bits), O (1 bit)

These bits can be laid out in a 16-byte array like this:

      Byte Offset
    0123456789ABCDEF
Bit
  7 HHHHHHHHHIIIIIII
  6 GGGGGGGGGGJJJJJJ
  5 FFFFFFFFFFFKKKKK
  4 EEEEEEEEEEEELLLL
  3 DDDDDDDDDDDDDMMM
  2 CCCCCCCCCCCCCCNN
  1 BBBBBBBBBBBBBBBO
  0 AAAAAAAAAAAAAAAA

For example, to test bit X of A, we evaluate ((bits[X] & 1) != 0), or to
test bit X of I, we evaluate ((bits[9 + X] & 0x80) != 0). This can be done
in 1-2 machine instructions on x86, or 4-6 instructions on ARM.

This uses the LPT multiprocessor scheduling algorithm to lay out the bits
efficiently.

Saves ~450KB of instructions in a recent build of Chromium.

Differential Revision: http://reviews.llvm.org/D7954

llvm-svn: 231043

1 files changed, 75 insertions, 15 deletions

diff --git a/llvm/unittests/Transforms/IPO/LowerBitSets.cpp b/llvm/unittests/Transforms/IPO/LowerBitSets.cpp
index 26a42528378..613a15fe0d0 100644
--- a/llvm/unittests/Transforms/IPO/LowerBitSets.cpp
+++ b/llvm/unittests/Transforms/IPO/LowerBitSets.cpp
@@ -15,27 +15,39 @@ using namespace llvm;
 TEST(LowerBitSets, BitSetBuilder) {
   struct {
     std::vector<uint64_t> Offsets;
-    std::vector<uint8_t> Bits;
+    std::set<uint64_t> Bits;
     uint64_t ByteOffset;
     uint64_t BitSize;
     unsigned AlignLog2;
     bool IsSingleOffset;
     bool IsAllOnes;
   } BSBTests[] = {
-      {{}, {0}, 0, 1, 0, false, false},
-      {{0}, {1}, 0, 1, 0, true, true},
-      {{4}, {1}, 4, 1, 0, true, true},
-      {{37}, {1}, 37, 1, 0, true, true},
-      {{0, 1}, {3}, 0, 2, 0, false, true},
-      {{0, 4}, {3}, 0, 2, 2, false, true},
-      {{0, uint64_t(1) << 33}, {3}, 0, 2, 33, false, true},
-      {{3, 7}, {3}, 3, 2, 2, false, true},
-      {{0, 1, 7}, {131}, 0, 8, 0, false, false},
-      {{0, 2, 14}, {131}, 0, 8, 1, false, false},
-      {{0, 1, 8}, {3, 1}, 0, 9, 0, false, false},
-      {{0, 2, 16}, {3, 1}, 0, 9, 1, false, false},
-      {{0, 1, 2, 3, 4, 5, 6, 7}, {255}, 0, 8, 0, false, true},
-      {{0, 1, 2, 3, 4, 5, 6, 7, 8}, {255, 1}, 0, 9, 0, false, true},
+      {{}, {}, 0, 1, 0, false, false},
+      {{0}, {0}, 0, 1, 0, true, true},
+      {{4}, {0}, 4, 1, 0, true, true},
+      {{37}, {0}, 37, 1, 0, true, true},
+      {{0, 1}, {0, 1}, 0, 2, 0, false, true},
+      {{0, 4}, {0, 1}, 0, 2, 2, false, true},
+      {{0, uint64_t(1) << 33}, {0, 1}, 0, 2, 33, false, true},
+      {{3, 7}, {0, 1}, 3, 2, 2, false, true},
+      {{0, 1, 7}, {0, 1, 7}, 0, 8, 0, false, false},
+      {{0, 2, 14}, {0, 1, 7}, 0, 8, 1, false, false},
+      {{0, 1, 8}, {0, 1, 8}, 0, 9, 0, false, false},
+      {{0, 2, 16}, {0, 1, 8}, 0, 9, 1, false, false},
+      {{0, 1, 2, 3, 4, 5, 6, 7},
+       {0, 1, 2, 3, 4, 5, 6, 7},
+       0,
+       8,
+       0,
+       false,
+       true},
+      {{0, 1, 2, 3, 4, 5, 6, 7, 8},
+       {0, 1, 2, 3, 4, 5, 6, 7, 8},
+       0,
+       9,
+       0,
+       false,
+       true},
   };
 
   for (auto &&T : BSBTests) {
@@ -93,3 +105,51 @@ TEST(LowerBitSets, GlobalLayoutBuilder) {
     EXPECT_EQ(T.WantLayout, ComputedLayout);
   }
 }
+
+TEST(LowerBitSets, ByteArrayBuilder) {
+  struct BABAlloc {
+    std::set<uint64_t> Bits;
+    uint64_t BitSize;
+    uint64_t WantByteOffset;
+    uint8_t WantMask;
+  };
+
+  struct {
+    std::vector<BABAlloc> Allocs;
+    std::vector<uint8_t> WantBytes;
+  } BABTests[] = {
+      {{{{0}, 1, 0, 1}, {{0}, 1, 0, 2}}, {3}},
+      {{{{0}, 16, 0, 1},
+        {{1}, 15, 0, 2},
+        {{2}, 14, 0, 4},
+        {{3}, 13, 0, 8},
+        {{4}, 12, 0, 0x10},
+        {{5}, 11, 0, 0x20},
+        {{6}, 10, 0, 0x40},
+        {{7}, 9, 0, 0x80},
+        {{0}, 7, 9, 0x80},
+        {{0}, 6, 10, 0x40},
+        {{0}, 5, 11, 0x20},
+        {{0}, 4, 12, 0x10},
+        {{0}, 3, 13, 8},
+        {{0}, 2, 14, 4},
+        {{0}, 1, 15, 2}},
+       {1, 2, 4, 8, 0x10, 0x20, 0x40, 0x80, 0, 0x80, 0x40, 0x20, 0x10, 8, 4,
+        2}},
+  };
+
+  for (auto &&T : BABTests) {
+    ByteArrayBuilder BABuilder;
+
+    for (auto &&A : T.Allocs) {
+      uint64_t GotByteOffset;
+      uint8_t GotMask;
+
+      BABuilder.allocate(A.Bits, A.BitSize, GotByteOffset, GotMask);
+      EXPECT_EQ(A.WantByteOffset, GotByteOffset);
+      EXPECT_EQ(A.WantMask, GotMask);
+    }
+
+    EXPECT_EQ(T.WantBytes, BABuilder.Bytes);
+  }
+}