From: Jakub Jelinek <jakub@redhat.com> To: Aldy Hernandez <aldyh@redhat.com>, Andrew MacLeod <amacleod@redhat.com> Cc: gcc-patches@gcc.gnu.org Subject: [PATCH] range-op-float: Fix up -frounding-math frange_arithmetic +- handling [PR110755] Date: Mon, 24 Jul 2023 18:01:55 +0200 [thread overview] Message-ID: <ZL6gc8/8THf1gN18@tucnak> (raw) Hi! IEEE754 says that x + (-x) and x - x result in +0 in all rounding modes but rounding towards negative infinity, in which case the result is -0 for all finite x. x + x and x - (-x) if it is zero retain sign of x. Now, range_arithmetic implements the normal rounds to even rounding, and as the addition or subtraction in those cases is exact, we don't do any further rounding etc. and e.g. on the testcase below distilled from glibc compute a range [+0, +INF], which is fine for -fno-rounding-math or if we'd have a guarantee that those statements aren't executed with rounding towards negative infinity. I believe it is only +- which has this problematic behavior and I think it is best to deal with it in frange_arithmetic; if we know -frounding-math is on, it is x + (-x) or x - x and we are asked to round to negative infinity (i.e. want low bound rather than high bound), change +0 result to -0. Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk and after a while for 13.3? I'm afraid rushing this so late into 13.2... 2023-07-24 Jakub Jelinek <jakub@redhat.com> PR tree-optimization/110755 * range-op-float.cc (frange_arithmetic): Change +0 result to -0 for PLUS_EXPR or MINUS_EXPR if -frounding-math, inf is negative and it is exact op1 + (-op1) or op1 - op1. * gcc.dg/pr110755.c: New test. --- gcc/range-op-float.cc.jj 2023-07-23 19:32:20.832434105 +0200 +++ gcc/range-op-float.cc 2023-07-24 09:41:26.231030258 +0200 @@ -324,6 +324,24 @@ frange_arithmetic (enum tree_code code, bool inexact = real_arithmetic (&value, code, &op1, &op2); real_convert (&result, mode, &value); + /* When rounding towards negative infinity, x + (-x) and + x - x is -0 rather than +0 real_arithmetic computes. + So, when we are looking for lower bound (inf is negative), + use -0 rather than +0. */ + if (flag_rounding_math + && (code == PLUS_EXPR || code == MINUS_EXPR) + && !inexact + && real_iszero (&result) + && !real_isneg (&result) + && real_isneg (&inf)) + { + REAL_VALUE_TYPE op2a = op2; + if (code == PLUS_EXPR) + op2a.sign ^= 1; + if (real_isneg (&op1) == real_isneg (&op2a) && real_equal (&op1, &op2a)) + result.sign = 1; + } + // Be extra careful if there may be discrepancies between the // compile and runtime results. bool round = false; --- gcc/testsuite/gcc.dg/pr110755.c.jj 2023-07-21 10:34:05.037251433 +0200 +++ gcc/testsuite/gcc.dg/pr110755.c 2023-07-21 10:35:10.986326816 +0200 @@ -0,0 +1,29 @@ +/* PR tree-optimization/110755 */ +/* { dg-do run } */ +/* { dg-require-effective-target fenv } */ +/* { dg-require-effective-target hard_float } */ +/* { dg-options "-O2 -frounding-math" } */ + +#include <fenv.h> + +__attribute__((noipa)) float +foo (float x) +{ + if (x > 0.0) + { + x += 0x1p+23; + x -= 0x1p+23; + x = __builtin_fabsf (x); + } + return x; +} + +int +main () +{ +#ifdef FE_DOWNWARD + fesetround (FE_DOWNWARD); + if (__builtin_signbit (foo (0.5))) + __builtin_abort (); +#endif +} Jakub

next reply other threads:[~2023-07-24 16:02 UTC|newest]Thread overview:4+ messages / expand[flat|nested] mbox.gz Atom feed top2023-07-24 16:01 Jakub Jelinek [this message]2023-07-24 19:39 ` Joseph Myers 2023-07-24 19:45 ` Jakub Jelinek 2023-07-25 19:15 ` Aldy Hernandez

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