diff options
| author | Luc Van Oostenryck <luc.vanoostenryck@gmail.com> | 2021-04-18 00:45:43 +0200 |
|---|---|---|
| committer | Luc Van Oostenryck <luc.vanoostenryck@gmail.com> | 2021-04-18 17:18:57 +0200 |
| commit | 48eb2ca449b7f8e8e0684a2d92c8a1d0224861da (patch) | |
| tree | 18fd21d390a3d696e77fe5e78233fcfc4b019aa2 /simplify.c | |
| parent | eb4cdd21b7d0cedbbeff7f70e24473706ccce5a6 (diff) | |
| download | sparse-dev-48eb2ca449b7f8e8e0684a2d92c8a1d0224861da.tar.gz | |
canonicalize constant signed compares toward zero
Currently, signed compares against a constant are canonicalized
toward the smallest possible constant. So, the following
canonicalization are done:
x < 256 --> x <= 255
x < -2047 --> x <= -2048
This has two advantages:
1) it maximalizes the number of constants possible for a given bit size.
2) it allows to remove all < and all >=
But it has also a serious disadvantages: a simple comparison against
zero, like:
x >= 0
is canonicalized into:
x > -1
Which can be more costly for some architectures if translated as such ,
is also less readable than the version using 0 and is also sometimes
quite more complicated to match in some simplification patterns.
So, canonicalize it using 'towards 0' / using the smallest constant
in absolute value.
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Diffstat (limited to 'simplify.c')
| -rw-r--r-- | simplify.c | 34 |
1 files changed, 28 insertions, 6 deletions
@@ -1178,38 +1178,52 @@ static int simplify_compare_constant(struct instruction *insn, long long value) switch (insn->opcode) { case OP_SET_LT: + if (!value) + break; if (value == sign_bit(size)) // (x < SMIN) --> 0 return replace_with_pseudo(insn, value_pseudo(0)); if (value == sign_mask(size)) // (x < SMAX) --> (x != SMAX) return replace_opcode(insn, OP_SET_NE); if (value == sign_bit(size) + 1)// (x < SMIN + 1) --> (x == SMIN) return replace_binop_value(insn, OP_SET_EQ, sign_bit(size)); - changed |= replace_binop_value(insn, OP_SET_LE, (value - 1) & bits); + if (!(value & sign_bit(size))) + changed |= replace_binop_value(insn, OP_SET_LE, (value - 1) & bits); break; case OP_SET_LE: + if (!value) + break; if (value == sign_mask(size)) // (x <= SMAX) --> 1 return replace_with_pseudo(insn, value_pseudo(1)); if (value == sign_bit(size)) // (x <= SMIN) --> (x == SMIN) return replace_opcode(insn, OP_SET_EQ); if (value == sign_mask(size) - 1) // (x <= SMAX - 1) --> (x != SMAX) return replace_binop_value(insn, OP_SET_NE, sign_mask(size)); + if (value & sign_bit(size)) + changed |= replace_binop_value(insn, OP_SET_LT, (value + 1) & bits); break; case OP_SET_GE: + if (!value) + break; if (value == sign_bit(size)) // (x >= SMIN) --> 1 return replace_with_pseudo(insn, value_pseudo(1)); if (value == sign_mask(size)) // (x >= SMAX) --> (x == SMAX) return replace_opcode(insn, OP_SET_EQ); if (value == sign_bit(size) + 1)// (x >= SMIN + 1) --> (x != SMIN) return replace_binop_value(insn, OP_SET_NE, sign_bit(size)); - changed |= replace_binop_value(insn, OP_SET_GT, (value - 1) & bits); + if (!(value & sign_bit(size))) + changed |= replace_binop_value(insn, OP_SET_GT, (value - 1) & bits); break; case OP_SET_GT: + if (!value) + break; if (value == sign_mask(size)) // (x > SMAX) --> 0 return replace_with_pseudo(insn, value_pseudo(0)); if (value == sign_bit(size)) // (x > SMIN) --> (x != SMIN) return replace_opcode(insn, OP_SET_NE); if (value == sign_mask(size) - 1) // (x > SMAX - 1) --> (x == SMAX) return replace_binop_value(insn, OP_SET_EQ, sign_mask(size)); + if (value & sign_bit(size)) + changed |= replace_binop_value(insn, OP_SET_GE, (value + 1) & bits); break; case OP_SET_B: @@ -1271,8 +1285,10 @@ static int simplify_compare_constant(struct instruction *insn, long long value) if ((value & bits) != value) return replace_with_value(insn, 1); break; - case OP_SET_LE: + case OP_SET_LE: case OP_SET_LT: value = sign_extend(value, def->size); + if (insn->opcode == OP_SET_LT) + value -= 1; if (bits & sign_bit(def->size)) break; if (value < 0) @@ -1282,8 +1298,10 @@ static int simplify_compare_constant(struct instruction *insn, long long value) if (value == 0) return replace_opcode(insn, OP_SET_EQ); break; - case OP_SET_GT: + case OP_SET_GT: case OP_SET_GE: value = sign_extend(value, def->size); + if (insn->opcode == OP_SET_GE) + value -= 1; if (bits & sign_bit(def->size)) break; if (value < 0) @@ -1340,16 +1358,20 @@ static int simplify_compare_constant(struct instruction *insn, long long value) if (bits >= value) return replace_with_value(insn, 1); break; + case OP_SET_LT: + value -= 1; case OP_SET_LE: - value = sign_extend(value, def->size); if (bits & sign_bit(def->size)) { + value = sign_extend(value, def->size); if (value >= -1) return replace_with_value(insn, 1); } break; + case OP_SET_GE: + value -= 1; case OP_SET_GT: - value = sign_extend(value, def->size); if (bits & sign_bit(def->size)) { + value = sign_extend(value, def->size); if (value >= -1) return replace_with_value(insn, 0); } |