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([2804:1b3:a7c0:a715:c1a0:7281:6384:2ee9]) by smtp.gmail.com with ESMTPSA id k3-20020a05620a142300b0076f12fcb0easm3272722qkj.2.2023.11.06.05.27.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 06 Nov 2023 05:27:22 -0800 (PST) From: Adhemerval Zanella To: libc-alpha@sourceware.org, Bruno Haible Subject: [PATCH v2 3/7] x86: Do not raises floating-point exception traps on fesetexceptflag (BZ 30990) Date: Mon, 6 Nov 2023 10:27:09 -0300 Message-Id: <20231106132713.953501-4-adhemerval.zanella@linaro.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20231106132713.953501-1-adhemerval.zanella@linaro.org> References: <20231106132713.953501-1-adhemerval.zanella@linaro.org> MIME-Version: 1.0 X-Spam-Status: No, score=-12.5 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, GIT_PATCH_0, KAM_SHORT, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP, T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.30 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: libc-alpha-bounces+incoming=patchwork.ozlabs.org@sourceware.org From: Bruno Haible According to ISO C23 (7.6.4.4), fesetexcept is supposed to set floating-point exception flags without raising a trap (unlike feraiseexcept, which is supposed to raise a trap if feenableexcept was called with the appropriate argument). The flags can be set in the 387 unit or in the SSE unit. When we need to clear a flag, we need to do so in both units, due to the way fetestexcept is implemented. When we need to set a flag, it is sufficient to do it in the SSE unit, because that is guaranteed to not trap. However, on i386 CPUs that have only a 387 unit, set the flags in the 387, as long as this cannot trap. Co-authored-by: Adhemerval Zanella Reviewed-by: Carlos O'Donell --- math/test-fexcept-traps.c | 25 +++++++++++- sysdeps/i386/fpu/fsetexcptflg.c | 63 ++++++++++++++++++++----------- sysdeps/x86_64/fpu/fsetexcptflg.c | 24 +++++++----- 3 files changed, 79 insertions(+), 33 deletions(-) diff --git a/math/test-fexcept-traps.c b/math/test-fexcept-traps.c index 9b8f583ae6..6bfb5124da 100644 --- a/math/test-fexcept-traps.c +++ b/math/test-fexcept-traps.c @@ -19,6 +19,7 @@ #include #include #include +#include static int do_test (void) @@ -65,8 +66,28 @@ do_test (void) /* The test is that this does not cause exception traps. For architectures where setting the exception might result in traps the function should - return a nonzero value. */ - ret = fesetexceptflag (&saved, FE_ALL_EXCEPT); + return a nonzero value. + Also check if the function does not alter the exception mask. */ + { + int exc_before = fegetexcept (); + ret = fesetexceptflag (&saved, FE_ALL_EXCEPT); + int exc_after = fegetexcept (); + if (exc_before != exc_after) + { + puts ("fesetexceptflag (FE_ALL_EXCEPT) changed the exceptions mask"); + return 1; + } + } + + /* Execute some floating-point operations, since on some CPUs exceptions + triggers a trap only at the next floating-point instruction. */ + volatile double a = 1.0; + volatile double b = a + a; + math_force_eval (b); + volatile long double al = 1.0L; + volatile long double bl = al + al; + math_force_eval (bl); + if (ret != 0 && !EXCEPTION_SET_FORCES_TRAP) { puts ("fesetexceptflag failed"); diff --git a/sysdeps/i386/fpu/fsetexcptflg.c b/sysdeps/i386/fpu/fsetexcptflg.c index e724b7d6fd..480165cff9 100644 --- a/sysdeps/i386/fpu/fsetexcptflg.c +++ b/sysdeps/i386/fpu/fsetexcptflg.c @@ -17,42 +17,63 @@ . */ #include -#include -#include #include -#include int __fesetexceptflag (const fexcept_t *flagp, int excepts) { - fenv_t temp; + /* The flags can be set in the 387 unit or in the SSE unit. When we need to + clear a flag, we need to do so in both units, due to the way fetestexcept + is implemented. + When we need to set a flag, it is sufficient to do it in the SSE unit, + because that is guaranteed to not trap. However, on i386 CPUs that have + only a 387 unit, set the flags in the 387, as long as this cannot trap. */ - /* Get the current environment. We have to do this since we cannot - separately set the status word. */ - __asm__ ("fnstenv %0" : "=m" (*&temp)); + fenv_t temp; - temp.__status_word &= ~(excepts & FE_ALL_EXCEPT); - temp.__status_word |= *flagp & excepts & FE_ALL_EXCEPT; + excepts &= FE_ALL_EXCEPT; - /* Store the new status word (along with the rest of the environment. - Possibly new exceptions are set but they won't get executed unless - the next floating-point instruction. */ - __asm__ ("fldenv %0" : : "m" (*&temp)); + /* Get the current x87 FPU environment. We have to do this since we + cannot separately set the status word. + Note: fnstenv masks all floating-point exceptions until the fldenv + or fldcw below. */ + __asm__ ("fnstenv %0" : "=m" (*&temp)); - /* If the CPU supports SSE, we set the MXCSR as well. */ if (CPU_FEATURE_USABLE (SSE)) { - unsigned int xnew_exc; + unsigned int mxcsr; + + /* Clear relevant flags. */ + temp.__status_word &= ~(excepts & ~ *flagp); - /* Get the current MXCSR. */ - __asm__ ("stmxcsr %0" : "=m" (*&xnew_exc)); + /* Store the new status word (along with the rest of the environment). */ + __asm__ ("fldenv %0" : : "m" (*&temp)); - /* Set the relevant bits. */ - xnew_exc &= ~(excepts & FE_ALL_EXCEPT); - xnew_exc |= *flagp & excepts & FE_ALL_EXCEPT; + /* And now similarly for SSE. */ + __asm__ ("stmxcsr %0" : "=m" (*&mxcsr)); + + /* Clear or set relevant flags. */ + mxcsr ^= (mxcsr ^ *flagp) & excepts; /* Put the new data in effect. */ - __asm__ ("ldmxcsr %0" : : "m" (*&xnew_exc)); + __asm__ ("ldmxcsr %0" : : "m" (*&mxcsr)); + } + else + { + /* Clear or set relevant flags. */ + temp.__status_word ^= (temp.__status_word ^ *flagp) & excepts; + + if ((~temp.__control_word) & temp.__status_word & excepts) + { + /* Setting the exception flags may trigger a trap (at the next + floating-point instruction, but that does not matter). + ISO C 23 ยง 7.6.4.5 does not allow it. */ + __asm__ volatile ("fldcw %0" : : "m" (*&temp.__control_word)); + return -1; + } + + /* Store the new status word (along with the rest of the environment). */ + __asm__ ("fldenv %0" : : "m" (*&temp)); } /* Success. */ diff --git a/sysdeps/x86_64/fpu/fsetexcptflg.c b/sysdeps/x86_64/fpu/fsetexcptflg.c index a3ac1dea01..2ce2b509f2 100644 --- a/sysdeps/x86_64/fpu/fsetexcptflg.c +++ b/sysdeps/x86_64/fpu/fsetexcptflg.c @@ -22,30 +22,34 @@ int fesetexceptflag (const fexcept_t *flagp, int excepts) { + /* The flags can be set in the 387 unit or in the SSE unit. + When we need to clear a flag, we need to do so in both units, + due to the way fetestexcept() is implemented. + When we need to set a flag, it is sufficient to do it in the SSE unit, + because that is guaranteed to not trap. */ + fenv_t temp; unsigned int mxcsr; - /* XXX: Do we really need to set both the exception in both units? - Shouldn't it be enough to set only the SSE unit? */ + excepts &= FE_ALL_EXCEPT; /* Get the current x87 FPU environment. We have to do this since we cannot separately set the status word. */ __asm__ ("fnstenv %0" : "=m" (*&temp)); - temp.__status_word &= ~(excepts & FE_ALL_EXCEPT); - temp.__status_word |= *flagp & excepts & FE_ALL_EXCEPT; + /* Clear relevant flags. */ + temp.__status_word &= ~(excepts & ~ *flagp); - /* Store the new status word (along with the rest of the environment. - Possibly new exceptions are set but they won't get executed unless - the next floating-point instruction. */ + /* Store the new status word (along with the rest of the environment). */ __asm__ ("fldenv %0" : : "m" (*&temp)); - /* And now the same for SSE. */ + /* And now similarly for SSE. */ __asm__ ("stmxcsr %0" : "=m" (*&mxcsr)); - mxcsr &= ~(excepts & FE_ALL_EXCEPT); - mxcsr |= *flagp & excepts & FE_ALL_EXCEPT; + /* Clear or set relevant flags. */ + mxcsr ^= (mxcsr ^ *flagp) & excepts; + /* Put the new data in effect. */ __asm__ ("ldmxcsr %0" : : "m" (*&mxcsr)); /* Success. */