[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, 3 insertions, 4 deletions

diff --git a/llvm/lib/Target/X86/X86ShuffleDecodeConstantPool.cpp b/llvm/lib/Target/X86/X86ShuffleDecodeConstantPool.cpp
index 41ad0971461..c24b74f7480 100644
--- a/llvm/lib/Target/X86/X86ShuffleDecodeConstantPool.cpp
+++ b/llvm/lib/Target/X86/X86ShuffleDecodeConstantPool.cpp
@@ -77,8 +77,8 @@ static bool extractConstantMask(const Constant *C, unsigned MaskEltSizeInBits,
   RawMask.resize(NumMaskElts, 0);
 
   for (unsigned i = 0; i != NumMaskElts; ++i) {
-    APInt EltUndef = UndefBits.lshr(i * MaskEltSizeInBits);
-    EltUndef = EltUndef.zextOrTrunc(MaskEltSizeInBits);
+    unsigned BitOffset = i * MaskEltSizeInBits;
+    APInt EltUndef = UndefBits.extractBits(MaskEltSizeInBits, BitOffset);
 
     // Only treat the element as UNDEF if all bits are UNDEF, otherwise
     // treat it as zero.
@@ -88,8 +88,7 @@ static bool extractConstantMask(const Constant *C, unsigned MaskEltSizeInBits,
       continue;
     }
 
-    APInt EltBits = MaskBits.lshr(i * MaskEltSizeInBits);
-    EltBits = EltBits.zextOrTrunc(MaskEltSizeInBits);
+    APInt EltBits = MaskBits.extractBits(MaskEltSizeInBits, BitOffset);
     RawMask[i] = EltBits.getZExtValue();
   }