rieske / trans / 34111469960
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coverage: 94.413%. Remained the same
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fix: spill registers before a store through a pointer (#296)

A store through a pointer can reach any object whose address escaped,
but lvalueAssign emitted the store and left the register file alone.
A value read into a register before the store was still read from that
register afterwards:

    int x = 5, s = 0;
    int *p = &x;
    s += x;
    *p = 9;
    s += x;      /* 14 9 expected, 10 5 emitted */

    movslq -32(%rbp), %rbx    ; x -> %rbx (5)
    ...
    movl   %edx, (%rdi)       ; *p = 9, memory at -32(%rbp) is now 9
    addl   %ebx, %edx         ; %rbx still holds 5

Identical at -O0 and -O1, so it was never the IR passes. Systematic
rather than an edge case: a generator emitting read-then-store through
an aliasing pointer failed 450 of 500 runs.

Spilling before the store is the whole fix. Value::isStored() is
!assignedRegister, so every bound register counts as unstored and
spillGeneralPurposeRegisters() writes back and frees all of them;
lvalueAssign simply never called it. The order matters: spilling after
the store would write the pre-store value back over it.

The va_ paths and calls already spill and were unaffected, as are
stores through a struct member, an array element, a callee's pointer
argument, an sret destination, and a pointer to a pointer.

Costs 0.83% in emitted assembly over 60 programs, from spilling on
every pointer store rather than only for objects whose address is
known to have escaped. Computing that set in startProcedure, which
sees the whole body, would recover it; not worth the machinery yet.

15565 of 16486 relevant lines covered (94.41%)

155818.09 hits per line

Coverage Regressions

Lines Coverage ∆ File
10
96.66
 0.01% home/runner/work/trans/trans/src/codegen/StackMachine.cpp
Jobs
ID Job ID Ran Files Coverage
1 34111469960.1 274
94.41
 GitHub Action Run
Source Files on build 34111469960