I believe Micha stumbled over this as well. For stores to string literals we miss VDEFs and loads from STRING_CSTs miss VUSEs.
Bootstrap and regtest running on x86_64-unknown-linux-gnu. 2018-08-28 Richard Biener <rguent...@suse.de> PR tree-optimization/87117 * tree-ssa-sccvn.c (fully_constant_vn_reference_p): Exclude void which is is_gimple_reg_type by checking for COMPLETE_TYPE_P. * gcc.dg/pr87117-1.c: New testcase. Index: gcc/tree-ssa-sccvn.c =================================================================== --- gcc/tree-ssa-sccvn.c (revision 263906) +++ gcc/tree-ssa-sccvn.c (working copy) @@ -1408,6 +1408,7 @@ fully_constant_vn_reference_p (vn_refere /* Simplify reads from constants or constant initializers. */ else if (BITS_PER_UNIT == 8 + && COMPLETE_TYPE_P (ref->type) && is_gimple_reg_type (ref->type) && (!INTEGRAL_TYPE_P (ref->type) || TYPE_PRECISION (ref->type) % BITS_PER_UNIT == 0)) Index: gcc/testsuite/gcc.dg/pr87117-1.c =================================================================== --- gcc/testsuite/gcc.dg/pr87117-1.c (nonexistent) +++ gcc/testsuite/gcc.dg/pr87117-1.c (working copy) @@ -0,0 +1,21 @@ +/* { dg-do compile } */ +/* { dg-options "-O2 -fno-inline -fno-tree-dce" } */ + +int a, b, c; +long *d; +void fn1() +{ + for (; 0 < a;) + a++; +} +void fn3() +{ + for (; c; c++) + d[c] = 0; +} +void fn2() +{ + if (b) + fn3(); + fn1(); +}