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Don't fold unsigned divides of an constant and a negation (#4457)

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Negating an unsigned constant results in its two's complement which is
still interpreted as unsigned. For example -2u becomes 4294967294u.

Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/4456
This commit is contained in:
Nicolas Capens 2021-08-16 09:56:05 -04:00 committed by GitHub
parent 881001070a
commit 869a550d26
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GPG Key ID: 4AEE18F83AFDEB23
2 changed files with 29 additions and 6 deletions
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7
source/opt/folding_rules.cpp
View File

@ -967,12 +967,11 @@ FoldingRule MergeDivMulArithmetic() {
// Fold divides of a constant and a negation.
// Cases:
// (-x) / 2 = x / -2
// 2 / (-x) = 2 / -x
// 2 / (-x) = -2 / x
FoldingRule MergeDivNegateArithmetic() {
return [](IRContext* context, Instruction* inst,
const std::vector<const analysis::Constant*>& constants) {
assert(inst->opcode() == SpvOpFDiv || inst->opcode() == SpvOpSDiv ||
inst->opcode() == SpvOpUDiv);
assert(inst->opcode() == SpvOpFDiv || inst->opcode() == SpvOpSDiv);
analysis::ConstantManager* const_mgr = context->get_constant_mgr();
const analysis::Type* type =
context->get_type_mgr()->GetType(inst->type_id());
@ -2572,8 +2571,6 @@ void FoldingRules::AddFoldingRules() {
rules_[SpvOpStore].push_back(StoringUndef());
rules_[SpvOpUDiv].push_back(MergeDivNegateArithmetic());
rules_[SpvOpVectorShuffle].push_back(VectorShuffleFeedingShuffle());
rules_[SpvOpImageSampleImplicitLod].push_back(UpdateImageOperands());

28
test/opt/fold_test.cpp
View File

@ -5818,7 +5818,33 @@ INSTANTIATE_TEST_SUITE_P(MergeDivTest, MatchingInstructionFoldingTest,
"%5 = OpFDiv %float %4 %2\n" +
"OpReturn\n" +
"OpFunctionEnd\n",
5, true)
5, true),
// Test case 16: Do not merge udiv of snegate
// (-x) / 2u
InstructionFoldingCase<bool>(
Header() +
"%main = OpFunction %void None %void_func\n" +
"%main_lab = OpLabel\n" +
"%var = OpVariable %_ptr_uint Function\n" +
"%2 = OpLoad %uint %var\n" +
"%3 = OpSNegate %uint %2\n" +
"%4 = OpUDiv %uint %3 %uint_2\n" +
"OpReturn\n" +
"OpFunctionEnd\n",
4, false),
// Test case 17: Do not merge udiv of snegate
// 2u / (-x)
InstructionFoldingCase<bool>(
Header() +
"%main = OpFunction %void None %void_func\n" +
"%main_lab = OpLabel\n" +
"%var = OpVariable %_ptr_uint Function\n" +
"%2 = OpLoad %uint %var\n" +
"%3 = OpSNegate %uint %2\n" +
"%4 = OpUDiv %uint %uint_2 %3\n" +
"OpReturn\n" +
"OpFunctionEnd\n",
4, false)
));
INSTANTIATE_TEST_SUITE_P(MergeAddTest, MatchingInstructionFoldingTest,