125 lines
5.3 KiB
Diff
125 lines
5.3 KiB
Diff
commit e905212627350d54b58426214b5a54ddc852b0c9
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Author: Paul A. Clarke <pc@us.ibm.com>
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Date: Fri Aug 2 22:47:57 2019 -0400
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[powerpc] SET_RESTORE_ROUND improvements
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SET_RESTORE_ROUND uses libc_feholdsetround_ppc_ctx and
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libc_feresetround_ppc_ctx to bracket a block of code where the floating point
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rounding mode must be set to a certain value.
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For the *prologue*, libc_feholdsetround_ppc_ctx is used and performs:
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1. Read/save FPSCR.
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2. Create new value for FPSCR with new rounding mode and enables cleared.
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3. If new value is different than current value,
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a. If transitioning from a state where some exceptions enabled,
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enter "ignore exceptions / non-stop" mode.
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b. Write new value to FPSCR.
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c. Put a mark on the wall indicating the FPSCR was changed.
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(1) uses the 'mffs' instruction. On POWER9, the lighter weight 'mffsl'
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instruction can be used, but it doesn't return all of the bits in the FPSCR.
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fegetenv_status uses 'mffsl' on POWER9, 'mffs' otherwise, and can thus be
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used instead of fegetenv_register.
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(3b) uses 'mtfsf 0b11111111' to write the entire FPSCR, so it must
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instead use 'mtfsf 0b00000011' to write just the enables and the mode,
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because some of the rest of the bits are not valid if 'mffsl' was used.
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fesetenv_mode uses 'mtfsf 0b00000011' on POWER9, 'mtfsf 0b11111111'
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otherwise.
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For the *epilogue*, libc_feresetround_ppc_ctx checks the mark on the wall, then
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calls libc_feresetround_ppc, which just calls __libc_femergeenv_ppc with
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parameters such that it performs:
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1. Retreive saved value of FPSCR, saved in prologue above.
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2. Read FPSCR.
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3. Create new value of FPSCR where:
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- Summary bits and exception indicators = current OR saved.
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- Rounding mode and enables = saved.
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- Status bits = current.
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4. If transitioning from some exceptions enabled to none,
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enter "ignore exceptions / non-stop" mode.
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5. If transitioning from no exceptions enabled to some,
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enter "catch exceptions" mode.
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6. Write new value to FPSCR.
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The summary bits are hardwired to the exception indicators, so there is no
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need to restore any saved summary bits.
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The exception indicator bits, which are sticky and remain set unless
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explicitly cleared, would only need to be restored if the code block
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might explicitly clear any of them. This is certainly not expected.
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So, the only bits that need to be restored are the enables and the mode.
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If it is the case that only those bits are to be restored, there is no need to
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read the FPSCR. Steps (2) and (3) are unnecessary, and step (6) only needs to
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write the bits being restored.
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We know we are transitioning out of "ignore exceptions" mode, so step (4) is
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unnecessary, and in step (6), we only need to check the state we are
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entering.
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diff --git a/sysdeps/powerpc/fpu/fenv_private.h b/sysdeps/powerpc/fpu/fenv_private.h
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index 945ab98018450092..b0149aa243e69f5a 100644
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--- a/sysdeps/powerpc/fpu/fenv_private.h
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+++ b/sysdeps/powerpc/fpu/fenv_private.h
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@@ -132,7 +132,17 @@ libc_fesetenv_ppc (const fenv_t *envp)
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static __always_inline void
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libc_feresetround_ppc (fenv_t *envp)
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{
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- __libc_femergeenv_ppc (envp, _FPU_MASK_TRAPS_RN, _FPU_MASK_FRAC_INEX_RET_CC);
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+ fenv_union_t new = { .fenv = *envp };
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+
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+ /* If the old env has no enabled exceptions and the new env has any enabled
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+ exceptions, then unmask SIGFPE in the MSR FE0/FE1 bits. This will put the
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+ hardware into "precise mode" and may cause the FPU to run slower on some
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+ hardware. */
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+ if ((new.l & _FPU_ALL_TRAPS) != 0)
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+ (void) __fe_nomask_env_priv ();
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+
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+ /* Atomically enable and raise (if appropriate) exceptions set in `new'. */
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+ fesetenv_mode (new.fenv);
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}
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static __always_inline int
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@@ -176,9 +186,30 @@ libc_feholdsetround_ppc_ctx (struct rm_ctx *ctx, int r)
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{
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fenv_union_t old, new;
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+ old.fenv = fegetenv_status ();
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+
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+ new.l = (old.l & ~(FPSCR_ENABLES_MASK|FPSCR_RN_MASK)) | r;
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+
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+ ctx->env = old.fenv;
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+ if (__glibc_unlikely (new.l != old.l))
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+ {
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+ if ((old.l & _FPU_ALL_TRAPS) != 0)
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+ (void) __fe_mask_env ();
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+ fesetenv_mode (new.fenv);
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+ ctx->updated_status = true;
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+ }
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+ else
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+ ctx->updated_status = false;
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+}
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+
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+static __always_inline void
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+libc_feholdsetround_noex_ppc_ctx (struct rm_ctx *ctx, int r)
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+{
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+ fenv_union_t old, new;
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+
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old.fenv = fegetenv_register ();
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- new.l = (old.l & _FPU_MASK_TRAPS_RN) | r;
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+ new.l = (old.l & ~(FPSCR_ENABLES_MASK|FPSCR_RN_MASK)) | r;
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ctx->env = old.fenv;
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if (__glibc_unlikely (new.l != old.l))
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@@ -218,6 +249,9 @@ libc_feresetround_ppc_ctx (struct rm_ctx *ctx)
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#define libc_feholdsetround_ctx libc_feholdsetround_ppc_ctx
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#define libc_feholdsetroundf_ctx libc_feholdsetround_ppc_ctx
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#define libc_feholdsetroundl_ctx libc_feholdsetround_ppc_ctx
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+#define libc_feholdsetround_noex_ctx libc_feholdsetround_noex_ppc_ctx
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+#define libc_feholdsetround_noexf_ctx libc_feholdsetround_noex_ppc_ctx
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+#define libc_feholdsetround_noexl_ctx libc_feholdsetround_noex_ppc_ctx
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#define libc_feresetround_ctx libc_feresetround_ppc_ctx
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#define libc_feresetroundf_ctx libc_feresetround_ppc_ctx
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#define libc_feresetroundl_ctx libc_feresetround_ppc_ctx
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