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1 // Copyright 2019 the V8 project authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
4 
5 #include <stdint.h>
6 #include <limits>
7 
8 #include "src/base/build_config.h"
9 
10 #if !defined(UNDEFINED_SANITIZER) || !defined(V8_TARGET_ARCH_32_BIT)
11 #error "This file is only needed for 32-bit UBSan builds."
12 #endif
13 
14 // Compiling with -fsanitize=undefined on 32-bit platforms requires __mulodi4
15 // to be available. Usually it comes from libcompiler_rt, which our build
16 // doesn't provide, so here is a custom implementation (inspired by digit_mul
17 // in src/objects/bigint.cc).
__mulodi4(int64_t a,int64_t b,int * overflow)18 extern "C" int64_t __mulodi4(int64_t a, int64_t b, int* overflow) {
19   // Multiply in 32-bit chunks.
20   // For inputs [AH AL]*[BH BL], the result is:
21   //
22   //            [AL*BL]  // r_low
23   //    +    [AL*BH]     // r_mid1
24   //    +    [AH*BL]     // r_mid2
25   //    + [AH*BH]        // r_high
26   //    = [R4 R3 R2 R1]  // high = [R4 R3], low = [R2 R1]
27   //
28   // Where of course we must be careful with carries between the columns.
29   uint64_t a_low = a & 0xFFFFFFFFu;
30   uint64_t a_high = static_cast<uint64_t>(a) >> 32;
31   uint64_t b_low = b & 0xFFFFFFFFu;
32   uint64_t b_high = static_cast<uint64_t>(b) >> 32;
33 
34   uint64_t r_low = a_low * b_low;
35   uint64_t r_mid1 = a_low * b_high;
36   uint64_t r_mid2 = a_high * b_low;
37   uint64_t r_high = a_high * b_high;
38 
39   uint64_t result1 = r_low + (r_mid1 << 32);
40   if (result1 < r_low) r_high++;
41   uint64_t result2 = result1 + (r_mid2 << 32);
42   if (result2 < result1) r_high++;
43   r_high += (r_mid1 >> 32) + (r_mid2 >> 32);
44   int64_t result = static_cast<int64_t>(result2);
45   uint64_t result_sign = (result >> 63);
46   uint64_t expected_result_sign = (a >> 63) ^ (b >> 63);
47 
48   *overflow = (r_high > 0 || result_sign != expected_result_sign) ? 1 : 0;
49   return result;
50 }
51