[RFC] asm goto

[Richard Henderson <rth at redhat dot com>]

Some prodding from the kernel folk here at Red Hat has lead me to 
implementing a mechanism by which an asm statement can branch to a C label.

There is a major restriction on this form of asm statement, in that we 
cannot support output reloads on jump instructions, and therefore we 
cannot support outputs from the branching asm.

However, the usage case from the kernel folk doesn't actually need 
outputs.  They want to perform some code patching for (essentially) zero 
cost tracing.  Their use case looks vaguely like

+#define TRACE1(NUM)                         \
+  do {                                      \
+    asm goto ("0: nop;"                     \
+              ".pushsection trace_table;"   \
+              ".long 0b, %l0;"              \
+              ".popsection"                 \
+             : : : : trace#NUM);           \
+    if (0) { trace#NUM: trace(); }          \
+  } while (0)
+#define TRACE  TRACE1(__COUNTER__)

where instances of the TRACE macro get sprinkled about the kernel.  It's 
expected that the compiler rearranges the blocks here such that the 
straight-line code path normally consists of only the nop insn.  But 
we've recorded enough information in the trace_table section to allow 
that nop insn to be patched to be direct jump to the trace#NUM label, 
which calls the trace function followed by a branch back into the 
straight-line code.

I've implemented this only for the C front-end at present, awaiting 
feedback that might change the syntax.  I'll adjust the C++ front end 
and add a bunch of parsing test cases following general approval of the 
idea.

So.  Comments?


r~

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