From 355a31f14724541904595965c87271d9f211e7ba Mon Sep 17 00:00:00 2001 From: Tianmin Shi Date: Sat, 15 Jan 2022 16:52:37 +0300 Subject: [PATCH] 8272014: Better array indexing Reviewed-by: yan --- .../src/cpu/x86/vm/c1_LIRGenerator_x86.cpp | 28 +++++++++++++++++-- 1 file changed, 26 insertions(+), 2 deletions(-) diff --git a/hotspot/src/cpu/x86/vm/c1_LIRGenerator_x86.cpp b/hotspot/src/cpu/x86/vm/c1_LIRGenerator_x86.cpp index 0e4c5c0dfc..9642392393 100644 --- a/hotspot/src/cpu/x86/vm/c1_LIRGenerator_x86.cpp +++ b/hotspot/src/cpu/x86/vm/c1_LIRGenerator_x86.cpp @@ -166,8 +166,32 @@ LIR_Address* LIRGenerator::emit_array_address(LIR_Opr array_opr, LIR_Opr index_o LIR_Address* addr; if (index_opr->is_constant()) { int elem_size = type2aelembytes(type); - addr = new LIR_Address(array_opr, - offset_in_bytes + index_opr->as_jint() * elem_size, type); +#ifdef _LP64 + jint index = index_opr->as_jint(); + jlong disp = offset_in_bytes + (jlong)(index) * elem_size; + if (disp > max_jint) { + // Displacement overflow. Cannot directly use instruction with 32-bit displacement for 64-bit addresses. + // Convert array index to long to do array offset computation with 64-bit values. + index_opr = new_register(T_LONG); + __ move(LIR_OprFact::longConst(index), index_opr); + addr = new LIR_Address(array_opr, index_opr, LIR_Address::scale(type), offset_in_bytes, type); + } else { + addr = new LIR_Address(array_opr, disp, type); + } +#else + // A displacement overflow can also occur for x86 but that is not a problem due to the 32-bit address range! + // Let's assume an array 'a' and an access with displacement 'disp'. When disp overflows, then "a + disp" will + // always be negative (i.e. underflows the 32-bit address range): + // Let N = 2^32: a + signed_overflow(disp) = a + disp - N. + // "a + disp" is always smaller than N. If an index was chosen which would point to an address beyond N, then + // range checks would catch that and throw an exception. Thus, a + disp < 0 holds which means that it always + // underflows the 32-bit address range: + // unsigned_underflow(a + signed_overflow(disp)) = unsigned_underflow(a + disp - N) + // = (a + disp - N) + N = a + disp + // This shows that we still end up at the correct address with a displacement overflow due to the 32-bit address + // range limitation. This overflow only needs to be handled if addresses can be larger as on 64-bit platforms. + addr = new LIR_Address(array_opr, offset_in_bytes + index_opr->as_jint() * elem_size, type); +#endif // _LP64 } else { #ifdef _LP64 if (index_opr->type() == T_INT) {