100 lines
3.5 KiB
Diff
100 lines
3.5 KiB
Diff
commit e724b0480bfa5ec04f39be8c7290330b495c59de
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Author: Florian Weimer <fweimer@redhat.com>
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Date: Fri Nov 4 10:18:03 2022 +0100
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libgcc: Special-case BFD ld unwind table encodings in find_fde_tail
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BFD ld (and the other linkers) only produce one encoding of these
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values. It is not necessary to use the general
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read_encoded_value_with_base decoding routine. This avoids the
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data-dependent branches in its implementation.
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libgcc/
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* unwind-dw2-fde-dip.c (find_fde_tail): Special-case encoding
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values actually used by BFD ld.
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diff --git a/libgcc/unwind-dw2-fde-dip.c b/libgcc/unwind-dw2-fde-dip.c
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index 25f2e44c5823cf64..d4821d7d19950f15 100644
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--- a/libgcc/unwind-dw2-fde-dip.c
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+++ b/libgcc/unwind-dw2-fde-dip.c
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@@ -396,10 +396,21 @@ find_fde_tail (_Unwind_Ptr pc,
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if (hdr->version != 1)
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return NULL;
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- p = read_encoded_value_with_base (hdr->eh_frame_ptr_enc,
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- base_from_cb_data (hdr->eh_frame_ptr_enc,
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- dbase),
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- p, &eh_frame);
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+ if (__builtin_expect (hdr->eh_frame_ptr_enc == (DW_EH_PE_sdata4
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+ | DW_EH_PE_pcrel), 1))
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+ {
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+ /* Specialized version of read_encoded_value_with_base, based on what
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+ BFD ld generates. */
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+ signed value __attribute__ ((mode (SI)));
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+ memcpy (&value, p, sizeof (value));
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+ p += sizeof (value);
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+ dbase = value; /* No adjustment because pcrel has base 0. */
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+ }
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+ else
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+ p = read_encoded_value_with_base (hdr->eh_frame_ptr_enc,
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+ base_from_cb_data (hdr->eh_frame_ptr_enc,
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+ dbase),
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+ p, &eh_frame);
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/* We require here specific table encoding to speed things up.
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Also, DW_EH_PE_datarel here means using PT_GNU_EH_FRAME start
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@@ -409,10 +420,20 @@ find_fde_tail (_Unwind_Ptr pc,
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{
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_Unwind_Ptr fde_count;
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- p = read_encoded_value_with_base (hdr->fde_count_enc,
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- base_from_cb_data (hdr->fde_count_enc,
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- dbase),
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- p, &fde_count);
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+ if (__builtin_expect (hdr->fde_count_enc == DW_EH_PE_udata4, 1))
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+ {
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+ /* Specialized version of read_encoded_value_with_base, based on
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+ what BFD ld generates. */
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+ unsigned value __attribute__ ((mode (SI)));
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+ memcpy (&value, p, sizeof (value));
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+ p += sizeof (value);
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+ fde_count = value;
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+ }
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+ else
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+ p = read_encoded_value_with_base (hdr->fde_count_enc,
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+ base_from_cb_data (hdr->fde_count_enc,
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+ dbase),
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+ p, &fde_count);
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/* Shouldn't happen. */
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if (fde_count == 0)
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return NULL;
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@@ -454,8 +475,25 @@ find_fde_tail (_Unwind_Ptr pc,
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f = (fde *) (table[mid].fde + data_base);
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f_enc = get_fde_encoding (f);
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f_enc_size = size_of_encoded_value (f_enc);
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- read_encoded_value_with_base (f_enc & 0x0f, 0,
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- &f->pc_begin[f_enc_size], &range);
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+
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+ /* BFD ld uses DW_EH_PE_sdata4 | DW_EH_PE_pcrel on non-FDPIC targets,
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+ so optimize for that.
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+
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+ This optimization is not valid for FDPIC targets. f_enc & 0x0f as
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+ passed to read_encoded_value_with_base masks away the base flags,
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+ but they are implicit for FDPIC. */
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+#ifndef __FDPIC__
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+ if (__builtin_expect (f_enc == (DW_EH_PE_sdata4 | DW_EH_PE_pcrel),
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+ 1))
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+ {
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+ signed value __attribute__ ((mode (SI)));
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+ memcpy (&value, &f->pc_begin[f_enc_size], sizeof (value));
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+ range = value;
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+ }
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+ else
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+#endif
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+ read_encoded_value_with_base (f_enc & 0x0f, 0,
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+ &f->pc_begin[f_enc_size], &range);
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_Unwind_Ptr func = table[mid].initial_loc + data_base;
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if (pc < table[mid].initial_loc + data_base + range)
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{
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