https://bugs.kde.org/show_bug.cgi?id=511931
Bug ID: 511931
Summary: vex: improve register allocator when non-virtual
registers are assigned in insn selection
Classification: Developer tools
Product: valgrind
Version First unspecified
Reported In:
Platform: Other
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: vex
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ---
It seems that the register allocator does not like when insn selection assigns
non-virtual registers. I observe unneeded register-register copies in that
case.
This happens on s390 where 128-bit values for BFP and DFP need to be located in
a register pair (p,q) such that regno(q) == regno(p) + 2
The reg allocator does not know about that requirement and hence non-virtual
register
pairs are assigned during insn selection to avoid SIGILLs.
To demonstrate:
auxprogs/s390-runone -t -i "sqxbr %f0,%f1" > foo.c
auxprogs/s390-runone -b foo.c
vg-in-place --tool=none --trace-notbelow=0 --trace-flags=00000110 ./foo >&
foo.trace
The register allocated code has this:
v-load %f7,80(%r13)
v-load %f6,112(%r13)
v-move %f13,%f7
v-move %f15,%f6
v-fsqrt %f12,%f13
v-move %f7,%f12
v-move %f6,%f14
v-store %f7,64(%r13)
v-store %f6,96(%r13)
wasting two registers (f6, f7) just to move things around.
It should be:
v-load %f13,80(%r13)
v-load %f15,112(%r13)
v-fsqrt %f12,%f13
v-store %f12,64(%r13)
v-store %f14,96(%r13)
It gets significantly worse when computations get a bit more complex. E.g. try
adding 3 numbers..
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