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Prevent some invalid associativity in to_float implementation for avx… #1265

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Merged
serge-sans-paille merged 1 commit into from
Feb 27, 2026
+22 −0
Merged

Prevent some invalid associativity in to_float implementation for avx… #1265

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11 changes: 11 additions & 0 deletions include/xsimd/arch/xsimd_avx2.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -528,6 +528,13 @@ namespace xsimd
0xFFFF, 0xFFFF, 0x0000, 0x0000, 0xFFFF, 0xFFFF, 0x0000, 0x0000);
__m256i xL = _mm256_or_si256(_mm256_and_si256(mask, x), _mm256_andnot_si256(mask, _mm256_castpd_si256(_mm256_set1_pd(0x0010000000000000)))); // 2^52
__m256d f = _mm256_sub_pd(_mm256_castsi256_pd(xH), _mm256_set1_pd(19342813118337666422669312.)); // 2^84 + 2^52
// With -ffast-math, the compiler may reassociate (xH-C)+xL into
// xH+(xL-C). Since xL<<C this causes catastrophic cancellation.
// The asm barrier forces f into a register before the add, blocking
// the reorder. It emits zero instructions.
#if defined(__GNUC__)
__asm__ volatile("" : "+x"(f));
#endif
return _mm256_add_pd(f, _mm256_castsi256_pd(xL));
}

Expand All @@ -543,6 +550,10 @@ namespace xsimd
0xFFFF, 0xFFFF, 0xFFFF, 0x0000, 0xFFFF, 0xFFFF, 0xFFFF, 0x0000);
__m256i xL = _mm256_or_si256(_mm256_and_si256(mask, x), _mm256_andnot_si256(mask, _mm256_castpd_si256(_mm256_set1_pd(0x0010000000000000)))); // 2^52
__m256d f = _mm256_sub_pd(_mm256_castsi256_pd(xH), _mm256_set1_pd(442726361368656609280.)); // 3*2^67 + 2^52
// See above: prevent -ffast-math from reassociating (xH-C)+xL.
#if defined(__GNUC__)
__asm__ volatile("" : "+x"(f));
#endif
return _mm256_add_pd(f, _mm256_castsi256_pd(xL));
}
}
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11 changes: 11 additions & 0 deletions include/xsimd/arch/xsimd_sse4_1.hpp
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Expand Up @@ -53,6 +53,13 @@ namespace xsimd
xH = _mm_add_epi64(xH, _mm_castpd_si128(_mm_set1_pd(442721857769029238784.))); // 3*2^67
__m128i xL = _mm_blend_epi16(x, _mm_castpd_si128(_mm_set1_pd(0x0010000000000000)), 0x88); // 2^52
__m128d f = _mm_sub_pd(_mm_castsi128_pd(xH), _mm_set1_pd(442726361368656609280.)); // 3*2^67 + 2^52
// With -ffast-math, the compiler may reassociate (xH-C)+xL into
// xH+(xL-C). Since xL<<C this causes catastrophic cancellation.
// The asm barrier forces f into a register before the add, blocking
// the reorder. It emits zero instructions.
#if defined(__GNUC__)
__asm__ volatile("" : "+x"(f));
#endif
return _mm_add_pd(f, _mm_castsi128_pd(xL));
}

Expand All @@ -64,6 +71,10 @@ namespace xsimd
xH = _mm_or_si128(xH, _mm_castpd_si128(_mm_set1_pd(19342813113834066795298816.))); // 2^84
__m128i xL = _mm_blend_epi16(x, _mm_castpd_si128(_mm_set1_pd(0x0010000000000000)), 0xcc); // 2^52
__m128d f = _mm_sub_pd(_mm_castsi128_pd(xH), _mm_set1_pd(19342813118337666422669312.)); // 2^84 + 2^52
// See above: prevent -ffast-math from reassociating (xH-C)+xL.
#if defined(__GNUC__)
__asm__ volatile("" : "+x"(f));
#endif
return _mm_add_pd(f, _mm_castsi128_pd(xL));
}
}
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