Author: vedantk Date: Wed Jan 3 15:11:32 2018 New Revision: 321771 URL: http://llvm.org/viewvc/llvm-project?rev=321771&view=rev Log: [CGBuiltin] Handle unsigned mul overflow properly (PR35750)
r320902 fixed the IRGen for some types of checked multiplications. It did not handle unsigned overflow correctly in the case where the signed operand is negative (PR35750). Eli pointed out that on overflow, the result must be equal to the unique value that is equivalent to the mathematically-correct result modulo two raised to the k power, where k is the number of bits in the result type. This patch fixes the specialized IRGen from r320902 accordingly. Testing: Apart from check-clang, I modified the test harness from r320902 to validate the results of all multiplications -- not just the ones which don't overflow: https://gist.github.com/vedantk/3eb9c88f82e5c32f2e590555b4af5081 llvm.org/PR35750, rdar://34963321 Differential Revision: https://reviews.llvm.org/D41717 Modified: cfe/trunk/lib/CodeGen/CGBuiltin.cpp cfe/trunk/test/CodeGen/builtins-overflow.c Modified: cfe/trunk/lib/CodeGen/CGBuiltin.cpp URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CGBuiltin.cpp?rev=321771&r1=321770&r2=321771&view=diff ============================================================================== --- cfe/trunk/lib/CodeGen/CGBuiltin.cpp (original) +++ cfe/trunk/lib/CodeGen/CGBuiltin.cpp Wed Jan 3 15:11:32 2018 @@ -915,7 +915,11 @@ EmitCheckedMixedSignMultiply(CodeGenFunc Overflow = CGF.Builder.CreateOr(Overflow, TruncOverflow); } - Result = CGF.Builder.CreateTrunc(UnsignedResult, ResTy); + // Negate the product if it would be negative in infinite precision. + Result = CGF.Builder.CreateSelect( + IsNegative, CGF.Builder.CreateNeg(UnsignedResult), UnsignedResult); + + Result = CGF.Builder.CreateTrunc(Result, ResTy); } assert(Overflow && Result && "Missing overflow or result"); Modified: cfe/trunk/test/CodeGen/builtins-overflow.c URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/CodeGen/builtins-overflow.c?rev=321771&r1=321770&r2=321771&view=diff ============================================================================== --- cfe/trunk/test/CodeGen/builtins-overflow.c (original) +++ cfe/trunk/test/CodeGen/builtins-overflow.c Wed Jan 3 15:11:32 2018 @@ -373,7 +373,9 @@ int test_mixed_sign_mull_overflow_unsign // CHECK-NEXT: [[NotNull:%.*]] = icmp ne i32 [[UnsignedResult]], 0 // CHECK-NEXT: [[Underflow:%.*]] = and i1 [[IsNeg]], [[NotNull]] // CHECK-NEXT: [[OFlow:%.*]] = or i1 [[UnsignedOFlow]], [[Underflow]] -// CHECK-NEXT: store i32 [[UnsignedResult]], i32* %{{.*}}, align 4 +// CHECK-NEXT: [[NegatedResult:%.*]] = sub i32 0, [[UnsignedResult]] +// CHECK-NEXT: [[Result:%.*]] = select i1 [[IsNeg]], i32 [[NegatedResult]], i32 [[UnsignedResult]] +// CHECK-NEXT: store i32 [[Result]], i32* %{{.*}}, align 4 // CHECK: br i1 [[OFlow]] unsigned result; @@ -432,7 +434,9 @@ long long test_mixed_sign_mulll_overflow // CHECK-NEXT: [[OVERFLOW_PRE_TRUNC:%.*]] = or i1 {{.*}}, [[UNDERFLOW]] // CHECK-NEXT: [[TRUNC_OVERFLOW:%.*]] = icmp ugt i64 [[UNSIGNED_RESULT]], 4294967295 // CHECK-NEXT: [[OVERFLOW:%.*]] = or i1 [[OVERFLOW_PRE_TRUNC]], [[TRUNC_OVERFLOW]] -// CHECK-NEXT: trunc i64 [[UNSIGNED_RESULT]] to i32 +// CHECK-NEXT: [[NEGATED:%.*]] = sub i64 0, [[UNSIGNED_RESULT]] +// CHECK-NEXT: [[RESULT:%.*]] = select i1 {{.*}}, i64 [[NEGATED]], i64 [[UNSIGNED_RESULT]] +// CHECK-NEXT: trunc i64 [[RESULT]] to i32 // CHECK-NEXT: store unsigned result; if (__builtin_mul_overflow(y, x, &result)) _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
