From dfd4c084f914539e605a1e34e7b3510dcd45f336 Mon Sep 17 00:00:00 2001 From: Samuel Grayson Date: Thu, 17 Oct 2019 13:15:57 -0500 Subject: [PATCH] ext: Fix undefined-behavior bug in bitshift If a small number or zero is passed in, fp64_exp could be very negative (-1000 for example). The intent of the line is to evaluate to zero in these cases, but what it actually did was bitshift right by 1000, which is undefined behavior (according to ubsan) that so happens to result in 0 on GCC/most architectures. This commit changes the code to check for cases where the bitshift is larger than the width of the integer. Change-Id: I8de4bd8ad170f0321d54689460de449b7f8fb60a Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21859 Reviewed-by: Andreas Sandberg Maintainer: Andreas Sandberg Tested-by: kokoro --- ext/fputils/fp80.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/ext/fputils/fp80.c b/ext/fputils/fp80.c index 6ba890480..05acfd91a 100644 --- a/ext/fputils/fp80.c +++ b/ext/fputils/fp80.c @@ -162,7 +162,10 @@ fp80_cvtfp64(fp80_t fp80) * as normals */ return build_fp64(sign, fp64_frac, fp64_exp); } else if (fp64_exp <= 0) { - uint64_t fp64_denormal_frac = fp64_frac >> (-fp64_exp); + uint64_t fp64_denormal_frac = -64 < fp64_exp + // -64 < fp_exp <= 0, so safe to bitshift by -fp_exp + ? fp64_frac >> (-fp64_exp) + : 0; /* Generate a denormal or zero */ return build_fp64(sign, fp64_denormal_frac, 0); } else { -- 2.30.2