[SimplifyLibCalls] Turn memchr(const, C, const) into a bitfield check.
strchr("123!", C) != nullptr is a common pattern to check if C is one
of 1, 2, 3 or !. If the largest element of the string is smaller than
the target's register size we can easily create a bitfield and just
do a simple test for set membership.
int foo(char C) { return strchr("123!", C) != nullptr; } now becomes
cmpl $64, %edi ## range check
sbbb %al, %al
movabsq $0xE000200000001, %rcx
btq %rdi, %rcx ## bit test
sbbb %cl, %cl
andb %al, %cl ## and the two conditions
andb $1, %cl
movzbl %cl, %eax ## returning an int
ret
(imho the backend should expand this into a series of branches, but
that's a different story)
The code is currently limited to bit fields that fit in a register, so
usually 64 or 32 bits. Sadly, this misses anything using alpha chars
or {}. This could be fixed by just emitting a i128 bit field, but that
can generate really ugly code so we have to find a better way. To some
degree this is also recreating switch lowering logic, but we can't
simply emit a switch instruction and thus change the CFG within
instcombine.
llvm-svn: 232902
diff --git a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
index 0d0b77a..f6cc431 100644
--- a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -762,11 +762,9 @@
if (LenC && LenC->isNullValue())
return Constant::getNullValue(CI->getType());
- // Check if all arguments are constants. If so, we can constant fold.
+ // From now on we need at least constant length and string.
StringRef Str;
- if (!CharC || !LenC ||
- !getConstantStringInfo(SrcStr, Str, /*Offset=*/0,
- /*TrimAtNul=*/false))
+ if (!LenC || !getConstantStringInfo(SrcStr, Str, 0, /*TrimAtNul=*/false))
return nullptr;
// Truncate the string to LenC. If Str is smaller than LenC we will still only
@@ -774,6 +772,54 @@
// return null if we don't find the char.
Str = Str.substr(0, LenC->getZExtValue());
+ // If the char is variable but the input str and length are not we can turn
+ // this memchr call into a simple bit field test. Of course this only works
+ // when the return value is only checked against null.
+ //
+ // It would be really nice to reuse switch lowering here but we can't change
+ // the CFG at this point.
+ //
+ // memchr("\r\n", C, 2) != nullptr -> (C & ((1 << '\r') | (1 << '\n'))) != 0
+ // after bounds check.
+ if (!CharC && !Str.empty() && isOnlyUsedInZeroEqualityComparison(CI)) {
+ unsigned char Max = *std::max_element(Str.begin(), Str.end());
+
+ // Make sure the bit field we're about to create fits in a register on the
+ // target.
+ // FIXME: On a 64 bit architecture this prevents us from using the
+ // interesting range of alpha ascii chars. We could do better by emitting
+ // two bitfields or shifting the range by 64 if no lower chars are used.
+ if (!DL.fitsInLegalInteger(Max + 1))
+ return nullptr;
+
+ // For the bit field use a power-of-2 type with at least 8 bits to avoid
+ // creating unnecessary illegal types.
+ unsigned char Width = NextPowerOf2(std::max((unsigned char)7, Max));
+
+ // Now build the bit field.
+ APInt Bitfield(Width, 0);
+ for (char C : Str)
+ Bitfield.setBit((unsigned char)C);
+ Value *BitfieldC = B.getInt(Bitfield);
+
+ // First check that the bit field access is within bounds.
+ Value *C = B.CreateZExtOrTrunc(CI->getArgOperand(1), BitfieldC->getType());
+ Value *Bounds = B.CreateICmp(ICmpInst::ICMP_ULT, C, B.getIntN(Width, Width),
+ "memchr.bounds");
+
+ // Create code that checks if the given bit is set in the field.
+ Value *Shl = B.CreateShl(B.getIntN(Width, 1ULL), C);
+ Value *Bits = B.CreateIsNotNull(B.CreateAnd(Shl, BitfieldC), "memchr.bits");
+
+ // Finally merge both checks and cast to pointer type. The inttoptr
+ // implicitly zexts the i1 to intptr type.
+ return B.CreateIntToPtr(B.CreateAnd(Bounds, Bits, "memchr"), CI->getType());
+ }
+
+ // Check if all arguments are constants. If so, we can constant fold.
+ if (!CharC)
+ return nullptr;
+
// Compute the offset.
size_t I = Str.find(CharC->getSExtValue() & 0xFF);
if (I == StringRef::npos) // Didn't find the char. memchr returns null.