https://bugs.llvm.org/show_bug.cgi?id=42173
Bug ID: 42173
Summary: llvm-mca markers interfere with loop vectorization
Product: libraries
Version: 8.0
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Loop Optimizer
Assignee: unassignedb...@nondot.org
Reporter: m...@marrone.nyc
CC: llvm-bugs@lists.llvm.org
The docs for llvm-mca
(https://llvm.org/docs/CommandGuide/llvm-mca.html#using-markers-to-analyze-specific-code-blocks)
suggest using inline assembly to mark the region that llvm-mca should examine,
e.g.
> for(size_t index = 0; index < count; index++)
> {
> __asm volatile("# LLVM-MCA-BEGIN");
> result += source[index];
> __asm volatile("# LLVM-MCA-END");
> }
However, these directives prevent auto-vectorization.
> <source>:8:3: remark: loop not vectorized: call instruction cannot be
> vectorized [-Rpass-analysis=loop-vectorize]
> __asm volatile("# LLVM-MCA-BEGIN sum_marked");
> ^
> <source>:6:2: remark: loop not vectorized: read with atomic ordering or
> volatile read [-Rpass-analysis=loop-vectorize]
> for (size_t index = 0; index < count; index++)
> ^
Compiler Explorer demo, on Clang 8.0.0: https://godbolt.org/z/NSQchu
We should be able to use llvm-mca markers without affecting optimization and
code generation.
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