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Fix Clang Tbprobe Miscompilation

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Recent changes to the Square enum (reducing it from int32_t to int8_t)
now allow the compiler to vectorize loops that were previously too wide
for targets below AVX-512. However, this vectorization which Clang
performs is not correct and causes a miscompilation.

Disable this vectorization.

This particular issue was noticable with Clang 15 and Clang 19,
on avx2 as well as applie-silicon.

Ref: #6063
Original Clang Issue: llvm/llvm-project#80494

First reported by #6528, though misinterpreted.

closes #6529

No functional change
This commit is contained in:
Disservin
2026-01-10 15:33:36 +01:00
committed by Joost VandeVondele
parent c27c1747e3
commit d39bfb61a2
1 changed files with 16 additions and 11 deletions
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src/syzygy/tbprobe.cpp
+16 -11
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@@ -709,15 +709,11 @@ int map_score(TBTable<DTZ>* entry, File f, int value, WDLScore wdl) {
return value + 1;
}
// A temporary fix for the compiler bug with AVX-512. (#4450)
#ifdef USE_AVX512
#if defined(__clang__) && defined(__clang_major__) && __clang_major__ >= 15
#define CLANG_AVX512_BUG_FIX __attribute__((optnone))
#endif
#endif
#ifndef CLANG_AVX512_BUG_FIX
#define CLANG_AVX512_BUG_FIX
// A temporary fix for the compiler bug with vectorization. (#4450)
#if defined(__clang__) && defined(__clang_major__) && __clang_major__ >= 15
#define DISABLE_CLANG_LOOP_VEC _Pragma("clang loop vectorize(disable)")
#else
#define DISABLE_CLANG_LOOP_VEC
#endif
// Compute a unique index out of a position and use it to probe the TB file. To
@@ -727,8 +723,7 @@ int map_score(TBTable<DTZ>* entry, File f, int value, WDLScore wdl) {
// idx = Binomial[1][s1] + Binomial[2][s2] + ... + Binomial[k][sk]
//
template<typename T, typename Ret = typename T::Ret>
CLANG_AVX512_BUG_FIX Ret
do_probe_table(const Position& pos, T* entry, WDLScore wdl, ProbeState* result) {
Ret do_probe_table(const Position& pos, T* entry, WDLScore wdl, ProbeState* result) {
Square squares[TBPIECES];
Piece pieces[TBPIECES];
@@ -812,8 +807,11 @@ do_probe_table(const Position& pos, T* entry, WDLScore wdl, ProbeState* result)
// Now we map again the squares so that the square of the lead piece is in
// the triangle A1-D1-D4.
if (file_of(squares[0]) > FILE_D)
{
DISABLE_CLANG_LOOP_VEC
for (int i = 0; i < size; ++i)
squares[i] = flip_file(squares[i]);
}
// Encode leading pawns starting with the one with minimum MapPawns[] and
// proceeding in ascending order.
@@ -832,19 +830,26 @@ do_probe_table(const Position& pos, T* entry, WDLScore wdl, ProbeState* result)
// In positions without pawns, we further flip the squares to ensure leading
// piece is below RANK_5.
if (rank_of(squares[0]) > RANK_4)
{
DISABLE_CLANG_LOOP_VEC
for (int i = 0; i < size; ++i)
squares[i] = flip_rank(squares[i]);
}
// Look for the first piece of the leading group not on the A1-D4 diagonal
// and ensure it is mapped below the diagonal.
DISABLE_CLANG_LOOP_VEC
for (int i = 0; i < d->groupLen[0]; ++i)
{
if (!off_A1H8(squares[i]))
continue;
if (off_A1H8(squares[i]) > 0) // A1-H8 diagonal flip: SQ_A3 -> SQ_C1
{
DISABLE_CLANG_LOOP_VEC
for (int j = i; j < size; ++j)
squares[j] = Square(((squares[j] >> 3) | (squares[j] << 3)) & 63);
}
break;
}