From 61541d7dfccdd2569d8726a74490f298068c9bd1 Mon Sep 17 00:00:00 2001 From: Peter Maydell Date: Mon, 22 Jul 2024 18:29:55 +0100 Subject: target/arm: Fix UMOPA/UMOPS of 16-bit values The UMOPA/UMOPS instructions are supposed to multiply unsigned 8 or 16 bit elements and accumulate the products into a 64-bit element. In the Arm ARM pseudocode, this is done with the usual infinite-precision signed arithmetic. However our implementation doesn't quite get it right, because in the DEF_IMOP_64() macro we do: sum += (NTYPE)(n >> 0) * (MTYPE)(m >> 0); where NTYPE and MTYPE are uint16_t or int16_t. In the uint16_t case, the C usual arithmetic conversions mean the values are converted to "int" type and the multiply is done as a 32-bit multiply. This means that if the inputs are, for example, 0xffff and 0xffff then the result is 0xFFFE0001 as an int, which is then promoted to uint64_t for the accumulation into sum; this promotion incorrectly sign extends the multiply. Avoid the incorrect sign extension by casting to int64_t before the multiply, so we do the multiply as 64-bit signed arithmetic, which is a type large enough that the multiply can never overflow into the sign bit. (The equivalent 8-bit operations in DEF_IMOP_32() are fine, because the 8-bit multiplies can never overflow into the sign bit of a 32-bit integer.) Cc: qemu-stable@nongnu.org Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2372 Signed-off-by: Peter Maydell Reviewed-by: Richard Henderson Message-id: 20240722172957.1041231-3-peter.maydell@linaro.org (cherry picked from commit ea3f5a90f036734522e9af3bffd77e69e9f47355) Signed-off-by: Michael Tokarev --- target/arm/tcg/sme_helper.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/target/arm/tcg/sme_helper.c b/target/arm/tcg/sme_helper.c index 5a6dd76489..f9001f5213 100644 --- a/target/arm/tcg/sme_helper.c +++ b/target/arm/tcg/sme_helper.c @@ -1146,10 +1146,10 @@ static uint64_t NAME(uint64_t n, uint64_t m, uint64_t a, uint8_t p, bool neg) \ uint64_t sum = 0; \ /* Apply P to N as a mask, making the inactive elements 0. */ \ n &= expand_pred_h(p); \ - sum += (NTYPE)(n >> 0) * (MTYPE)(m >> 0); \ - sum += (NTYPE)(n >> 16) * (MTYPE)(m >> 16); \ - sum += (NTYPE)(n >> 32) * (MTYPE)(m >> 32); \ - sum += (NTYPE)(n >> 48) * (MTYPE)(m >> 48); \ + sum += (int64_t)(NTYPE)(n >> 0) * (MTYPE)(m >> 0); \ + sum += (int64_t)(NTYPE)(n >> 16) * (MTYPE)(m >> 16); \ + sum += (int64_t)(NTYPE)(n >> 32) * (MTYPE)(m >> 32); \ + sum += (int64_t)(NTYPE)(n >> 48) * (MTYPE)(m >> 48); \ return neg ? a - sum : a + sum; \ } -- cgit v1.2.3