[compiler-rt] 55ec2ba - Fix PR47973: Addressing integer division edge case with INT_MIN

[Thomas Preud'homme via llvm-commits]

Author: Ayshe Kuran
Date: 2020-11-10T15:57:06Z
New Revision: 55ec2ba4bc36a0aa4c1d0bfddde312a120b05f1a

URL: https://github.com/llvm/llvm-project/commit/55ec2ba4bc36a0aa4c1d0bfddde312a120b05f1a
DIFF: https://github.com/llvm/llvm-project/commit/55ec2ba4bc36a0aa4c1d0bfddde312a120b05f1a.diff

LOG: Fix PR47973: Addressing integer division edge case with INT_MIN

Adjustment to integer division in int_div_impl.inc to avoid undefined behaviour that can occur as a result of having INT_MIN as one of the parameters.

Reviewed By: sepavloff

Differential Revision: https://reviews.llvm.org/D90218

Added: 
    

Modified: 
    compiler-rt/lib/builtins/int_div_impl.inc

Removed: 
    


################################################################################
diff  --git a/compiler-rt/lib/builtins/int_div_impl.inc b/compiler-rt/lib/builtins/int_div_impl.inc
index 5495b4b22274..dc1f97cbeae5 100644
--- a/compiler-rt/lib/builtins/int_div_impl.inc
+++ b/compiler-rt/lib/builtins/int_div_impl.inc
@@ -72,24 +72,24 @@ static __inline fixuint_t __umodXi3(fixuint_t n, fixuint_t d) {
 #ifdef COMPUTE_UDIV
 static __inline fixint_t __divXi3(fixint_t a, fixint_t b) {
   const int N = (int)(sizeof(fixint_t) * CHAR_BIT) - 1;
-  fixint_t s_a = a >> N;                   // s_a = a < 0 ? -1 : 0
-  fixint_t s_b = b >> N;                   // s_b = b < 0 ? -1 : 0
-  a = (a ^ s_a) - s_a;                     // negate if s_a == -1
-  b = (b ^ s_b) - s_b;                     // negate if s_b == -1
-  s_a ^= s_b;                              // sign of quotient
-  return (COMPUTE_UDIV(a, b) ^ s_a) - s_a; // negate if s_a == -1
+  fixint_t s_a = a >> N;                            // s_a = a < 0 ? -1 : 0
+  fixint_t s_b = b >> N;                            // s_b = b < 0 ? -1 : 0
+  fixuint_t a_u = (fixuint_t)(a ^ s_a) + (-s_a);    // negate if s_a == -1
+  fixuint_t b_u = (fixuint_t)(b ^ s_b) + (-s_b);    // negate if s_b == -1
+  s_a ^= s_b;                                       // sign of quotient
+  return (COMPUTE_UDIV(a_u, b_u) ^ s_a) + (-s_a);   // negate if s_a == -1
 }
 #endif // COMPUTE_UDIV
 
 #ifdef ASSIGN_UMOD
 static __inline fixint_t __modXi3(fixint_t a, fixint_t b) {
   const int N = (int)(sizeof(fixint_t) * CHAR_BIT) - 1;
-  fixint_t s = b >> N;    // s = b < 0 ? -1 : 0
-  b = (b ^ s) - s;        // negate if s == -1
-  s = a >> N;             // s = a < 0 ? -1 : 0
-  a = (a ^ s) - s;        // negate if s == -1
+  fixint_t s = b >> N;                              // s = b < 0 ? -1 : 0
+  fixuint_t b_u = (fixuint_t)(b ^ s) + (-s);        // negate if s == -1
+  s = a >> N;                                       // s = a < 0 ? -1 : 0
+  fixuint_t a_u = (fixuint_t)(a ^ s) + (-s);        // negate if s == -1
   fixuint_t res;
-  ASSIGN_UMOD(res, a, b);
-  return (res ^ s) - s;   // negate if s == -1
+  ASSIGN_UMOD(res, a_u, b_u);
+  return (res ^ s) + (-s);                          // negate if s == -1
 }
 #endif // ASSIGN_UMOD