[StackColoring] Incorrect slot merging due to stackcoloring-lifetime-start-on-first-use #132085
Description
In the program below, the allocation a and allocation b have overlapping
live ranges, so it should be impossible to observe them having the same
address. Nevertheless StackColoring merges stack slots for a and b. This
is incorrect since addresses of those allocations might be captured by g.
define void @f() { start: %a = alloca [1000 x i8], align 1 %b = alloca [1000 x i8], align 1 call void @llvm.lifetime.start.p0(i64 1000, ptr %a) call void @llvm.lifetime.start.p0(i64 1000, ptr %b) call void @g(ptr %a) call void @llvm.lifetime.end.p0(i64 1000, ptr %a) call void @g(ptr %b) call void @llvm.lifetime.end.p0(i64 1000, ptr %b) ret void } declare void @g(ptr %n)$ llc-21 a.ll -print-before=stack-coloring -print-after=stack-coloring # Machine code for function f: IsSSA, TracksLiveness Frame Objects: fi#0: size=1000, align=1, at location [SP+8] fi#1: size=1000, align=1, at location [SP+8] <snip> # *** IR Dump After Merge disjoint stack slots (stack-coloring) ***: # Machine code for function f: IsSSA, TracksLiveness Frame Objects: fi#0: size=1000, align=1, at location [SP+8] fi#1: dead <snip>This is caused by stackcoloring-lifetime-start-on-first-use (the default) where
stack coloring shrinks live range towards first use.