[APInt] Add APInt::extractBits() method to extract APInt subrange

The current pattern for extract bits in range is typically:

Mask.lshr(BitOffset).trunc(SubSizeInBits);

Which can be particularly slow for large APInts (MaskSizeInBits > 64) as they require the allocation of memory for the temporary variable.

This is another of the compile time issues identified in PR32037 (see also D30265).

This patch adds the APInt::extractBits() helper method which avoids the temporary memory allocation.

Differential Revision: https://reviews.llvm.org/D30336

llvm-svn: 296141

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/unittests/ADT/APIntTest.cpp b/llvm/unittests/ADT/APIntTest.cpp
index 39a88cdca90..e8923b6ac01 100644
--- a/llvm/unittests/ADT/APIntTest.cpp
+++ b/llvm/unittests/ADT/APIntTest.cpp
@@ -1434,6 +1434,18 @@ TEST(APIntTest, isMask) {
   }
 }
 
+TEST(APIntTest, extractBits) {
+  APInt i32(32, 0x1234567);
+  EXPECT_EQ(0x3456, i32.extractBits(16, 4));
+
+  APInt i256(256, -16776961 /* 0xFFFFFFFFFFFFFFFFFFFFFFFFFF0000FF */, true);
+  EXPECT_EQ(255, i256.extractBits(16, 0));
+  EXPECT_EQ(127, i256.extractBits(16, 1));
+  EXPECT_EQ(-1, i256.extractBits(32, 64).getSExtValue());
+  EXPECT_EQ(-1, i256.extractBits(128, 128).getSExtValue());
+  EXPECT_EQ(-8388481, i256.extractBits(128, 1).getSExtValue());
+}
+
 #if defined(__clang__)
 // Disable the pragma warning from versions of Clang without -Wself-move
 #pragma clang diagnostic push