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47 lines
1.7 KiB
Diff
47 lines
1.7 KiB
Diff
http://www.linux-mips.org/archives/linux-mips/2004-09/msg00000.html
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Atsushi Nemoto <anemo@mba.ocn.ne.jp> writes:
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>/ Is this a get_user's problem or gcc's?/
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The latter. gcc is putting the empty asm:
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__asm__ ("":"=r" (__gu_val));
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into the delay slot of the call.
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Part of the problem is that gcc estimates the length of an asm to be the
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number of instruction separators + 1. This means that it estimates the
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asm above to be one instruction long, which is perhaps a little silly
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for an empty string.
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But the real problem is that gcc should never trust this estimate anyway,
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since each "instruction" could obviously be a multi-instruction macro.
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gcc should certainly never put asms into delay slots.
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FWIW, I don't think the bug is specific to 3.3 or 3.4. It could
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probably trigger for other gcc versions too. It is highly dependent
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on scheduling though.
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The attached 3.4.x patch fixes the problem there, but if you want to work
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around it for old versions, just avoid using empty asms if you can,
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or make them volatile if you can't.
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Of course, the problem isn't confined to empty asms. If you have an asm
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with a single, multi-instruction macro, gcc might try putting that in a
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delay slot too. You should at least get an assembler warning in that case.
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Richard
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--- gcc-3.4.1/gcc/config/mips/mips.md-orig 2004-09-02 10:38:36.000000000 -0500
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+++ gcc-3.4.1/gcc/config/mips/mips.md 2004-09-02 10:38:42.000000000 -0500
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@@ -251,7 +251,7 @@
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;; Can the instruction be put into a delay slot?
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(define_attr "can_delay" "no,yes"
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- (if_then_else (and (eq_attr "type" "!branch,call,jump")
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+ (if_then_else (and (eq_attr "type" "!branch,call,jump,multi")
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(and (eq_attr "hazard" "none")
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(eq_attr "single_insn" "yes")))
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(const_string "yes")
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