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[DebugInfo] Stop describing imms in TargetInstrInfo's describeLoadedValue() impl
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Authored by dstenb on Oct 17 2019, 6:47 AM.

Details

Summary

The default implementation of the describeLoadedValue() hook uses the
MoveImm property to determine if an instruction moves an immediate. If
an instruction has that property the function returns the second
operand, assuming that that is the immediate value the instruction
moves. As far as I can tell, the MoveImm property does not imply that
the second operand is the immediate value, nor that any other operand
necessarily holds the immediate value; it just means that the
instruction moves some immediate value.

One example where the second operand is not the immediate is SystemZ's
LZER instruction, which moves a zero immediate implicitly: $f0S = LZER.

That case triggered an out-of-bound assertion when getting the operand.
I have added a test case for that instruction.

Another example is ARM's MVN instruction, which holds the logical
bitwise NOT'd value of the immediate that is moved. For the following
reproducer:

extern void foo(int);
int main() { foo(-11); }

an incorrect call site value would be emitted:

$ clang --target=arm foo.c -O1 -g -Xclang -femit-debug-entry-values \
    -c -o - | ./build/bin/llvm-dwarfdump  - | \
    grep -A2 call_site_parameter

0x00000058:       DW_TAG_GNU_call_site_parameter
                    DW_AT_location (DW_OP_reg0 R0)
                    DW_AT_GNU_call_site_value (DW_OP_lit10)

Another example is the A2_combineii instruction on Hexagon which moves
two immediates to a super-register: $d0 = A2_combineii 20, 10.

Perhaps these are rare exceptions, and most MoveImm instructions hold
the immediate in the second operand, but in my opinion the default
implementation of the hook should only describe values that it can, by
some contract, guarantee are safe to describe, rather than leaving it up
to the targets to override the exceptions, as that can silently result
in incorrect call site values.

This patch adds X86's relevant move immediate instructions to the
target's hook implementation, so this commit should be a NFC for that
target. We need to do the same for ARM and AArch64.

Diff Detail

Event Timeline

dstenb created this revision.Oct 17 2019, 6:47 AM

LGTM! Handling each instruction separately is indeed more safer! Too bad that we can't relay much on MI generic flags such as this one.

vsk accepted this revision.Oct 17 2019, 2:02 PM

LGTM! Handling each instruction separately is indeed more safer! Too bad that we can't relay much on MI generic flags such as this one.

+ 1

This revision is now accepted and ready to land.Oct 17 2019, 2:02 PM

Thanks for the reviews! I'll land this shortly.

This revision was automatically updated to reflect the committed changes.