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Re: Postpone expanding va_arg until pass_stdarg


On 12-02-15 14:57, Michael Matz wrote:
Hi,

On Wed, 11 Feb 2015, Tom de Vries wrote:

My idea was to not generate temporaries and hence copies for
non-scalar types, but rather construct the "result" of va_arg directly
into the original LHS (that would then also trivially solve the
problem of nno-copyable types).

The copy mentioned here is of ap, not of the result of va_arg.

Whoops, I misread, yes.  Thanks.


Hi,

Btw, I'm not happy about the ap copies, but I haven't been able to get rid of them.

I'm not really sure yet why std_gimplify_va_arg_expr has a part
commented out. Michael, can you comment?

I think I did that because of SSA form.  The old sequence calculated

    vatmp = valist;
    vatmp = vatmp + boundary-1
    vatmp = vatmp & -boundary

(where the local variable in that function 'valist_tmp' is the tree
VAR_DECL 'vatmp') and then continue to use valist_tmp.  When in SSA form
the gimplifier will rewrite this into:

    vatmp_1 = valist;
    vatmp_2 = vatmp_1 + boundary-1
    vatmp_3 = vatmp_2 & -boundary

but the local valist_tmp variable will continue to be the VAR_DECL, not
the vatmp_3 ssa name.  Basically whenever one gimplifies a MODIFY_EXPR
while in SSA form it's suspicious.  So the new code simply build the
expression:

    ((valist + bound-1) & -bound)

gimplifies that into an rvalue (most probably an SSA name) and uses that
to go on generating code by making valist_tmp be that returned rvalue.

I think you'll find that removing that code will make the SSA verifier
scream or generate invalid code with -m32 when that hook is used.


Thanks for the detailed explanation. I'm not sure I understand the
problem well enough, so I'll try to trigger it and investigate.

Actually the above fails to mention what the real problem is :-)  The
problem is that the local variable valist_tmp will be used to generate
further code after the above expression is generated.  Without my patch it
will continue to point to the VAR_DECL, not to the SSA name that actually
holds the computed value in the generated code.


I have not been able to reproduce this problem (with a bootstrap build on x86_64 for all languages, and {unix/,unix/-m32} testing), so I've dropped this bit for now.

I've pushed the latest status to vries/expand-va-arg-at-pass-stdarg.

-ftree-stdarg-opt (the va_list_gpr/fpr_size optimization) has been renabled again. I needed patch "Always check phi-ops in optimize_va_list_gpr_fpr_size" for that.

With a similar bootstrap and reg-test as described above, there's only one failure left:
...
FAIL: gcc.dg/tree-ssa/stdarg-2.c scan-tree-dump stdarg "f15: va_list escapes 0, needs to save [148] GPR units and [1-9][0-9]* FPR units"
...
And this is due to the ap copy, which is classified as escape.

[ We're still expanding ifn_va_arg before the va_list_gpr/fpr_size optimization. ]

Thanks,
- Tom


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