http://llvm.org/bugs/show_bug.cgi?id=21205
Bug ID: 21205
Summary: [Reassociation w/fast-math] Assert when calling
getNeg() on undef of type float
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: ASSIGNED
Severity: normal
Priority: P
Component: Transformation Utilities
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected]
Classification: Unclassified
An assertion is hit in the Reassociation pass on x86 at -O2 -ffast-math with
the following code:
a, b;
fn1() {
switch (0)
case 0: {
float c;
b = a - c - 1;
}
}
The SROA introduces the following instruction:
%sub = fsub fast float %conv, undef
Complete function:
define i32 @fn1() {
entry:
%0 = load i32* @a, align 4, !tbaa !1
%conv = sitofp i32 %0 to float
%sub = fsub fast float %conv, undef
%sub1 = fsub fast float %sub, 1.000000e+00
%conv2 = fptosi float %sub1 to i32
store i32 %conv2, i32* @b, align 4, !tbaa !1
ret i32 undef
}
The reassociation pass is trying to canonicalize X-Y into X+(0-Y) to expose
more opportunities for reassociation. In this case, Y is undef with type
float, but getNeg() is being called, as opposed to getFNeg().
The logic in the pass looks like this:
static Value *NegateValue(Value *V, Instruction *BI) {
if (ConstantFP *C = dyn_cast<ConstantFP>(V))
return ConstantExpr::getFNeg(C);
if (Constant *C = dyn_cast<Constant>(V))
return ConstantExpr::getNeg(C);
...
I guess an undef of type float isn't a ConstantFP... ..continuing to
investigate a solution.
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