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Mitigate UB when subtracting pointers #124

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Mitigate UB when subtracting pointers #124

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3 changes: 3 additions & 0 deletions clang/lib/CodeGen/CGExprScalar.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -4447,6 +4447,9 @@ Value *ScalarExprEmitter::EmitSub(const BinOpInfo &op) {
divisor = CGF.CGM.getSize(elementSize);
}

// With -fms-kernel we emit normal sdiv to mitigate UB.
if (CGF.getLangOpts().Kernel)
return Builder.CreateSDiv(diffInChars, divisor, "sub.ptr.div");
// Otherwise, do a full sdiv. This uses the "exact" form of sdiv, since
// pointer difference in C is only defined in the case where both operands
// are pointing to elements of an array.
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13 changes: 13 additions & 0 deletions clang/test/CodeGen/MSKernel/subptr.c
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@@ -0,0 +1,13 @@
// RUN: %clang_cc1 -fms-kernel -triple x86_64-windows-msvc -O2 -emit-llvm %s -o - | FileCheck %s
// CHECK: define {{.*}} i64 @sub_ptrs(ptr noundef %p1, ptr noundef %p2)
// CHECK-NEXT: entry:
// CHECK-NEXT: %sub.ptr.lhs.cast = ptrtoint ptr %p1 to i64
// CHECK-NEXT: %sub.ptr.rhs.cast = ptrtoint ptr %p2 to i64
// CHECK-NEXT: %sub.ptr.sub = sub i64 %sub.ptr.lhs.cast, %sub.ptr.rhs.cast
// CHECK-NEXT: %sub.ptr.div = sdiv i64 %sub.ptr.sub, 4
// CHECK-NEXT: ret i64 %sub.ptr.div

long long sub_ptrs(int* p1, int* p2) {
return p1 - p2;
}